So, I don't have Arcana Unleashed yet, but we've gotten pretty extensive previews from the YouTube world - I'll be referring to the Dungeon Dudes' recent subclass ranking video (basically everything is awesome except maybe Enchanters and Transmuters, which are just fine.)
The Necromancer is a class fantasy that is kind of a tough nut to crack in TTRPGs. While in video games, you can often summon a ton of different monsters and let the computer automate their behavior (the Diablo Necromancer and WoW's Unholy Death Knight, as well as a similar vibe with its Demonology Warlock, each do this sort of thing) there's a real risk in a TTRPG space of struggling with managing such a large group of creatures.
Draw Steel, notably, released its first supplementary new class with the Summoner, which gave us a class entirely built around summoning lots of additional creatures, with subclasses that gave you the Necromancer, Demonologist, and also a Fey summoner and Elemental summoner. This class is intended for advanced players, though, and they warn new players off from trying to class as their first because of the complexity of controlling a lot of minions on the board (I imagine it might be easiest for players who have played as Director first to try their hand at it).
So, the question then is this: how well can we make a Necromancer work in D&D? Arcana Unleashed gives us the Necromancer Wizard, a revision of the School of Necromancy Wizard from the 2014 PHB.
I had assumed that one of the major changes would be building the class around Summon Undead rather than Animate Dead, as these Summon spells really streamline the whole "adding a creature to the battlefield" kind of gameplay. But evidently, instead, WotC has chosen to stick to Animate Dead, which uses the Monster Manual stats for Skeletons and Zombies, and is a concentration-free way to raise or maintain control over a group of undead.
Let's talk about a couple things first.
Skeletons in 5.5 have a +5 to hit, which is actually what we expect player characters to have at level 1, but will fall behind as you gain levels. Zombies only have a +3 to hit. So, how does controlling a lot of them at higher levels work and how does it feel?
In large numbers, even if they do fairly low damage (a zombie hits for 1d8+1, a Skeleton for 1d6+3,) if you can get enough of them, that damage starts to add up. Hit chance, of course, is also a big consideration. Against an AC of 16, which feels like something you could encounter at low and high levels, a Skeleton has a 50% hit chance and a Zombie has a 40% hit chance.
So, it's a numbers game - the more minions we can control, the better off we'll be. Remember that Animate Dead is concentration-free, though we'll need to spend a spell slot every day to maintain it (lest our undead minions turn on us or innocent NPCs nearby).
Off the bat, even before we can cast Animate Dead, Necromancers automatically get Find Familiar and get to use a Skeleton or Zombie as familiar options, and can make their familiar Undead (they can use other familiar forms and make them undead if they wish). Notably, there are new feats that empower familiars in various ways that a Necromancer might want to look into, such as Battle Familiar, which basically temporarily turns a familiar into something more like the Summon spells. But at this point, the familiar can also make attacks instead of you when you take the attack action (using its reaction). At this level, a Skeleton's Shortsword or Shortbow attack is pretty close to par with our use of True Strike (slightly less if we're using a light crossbow, greatclub, or two-handing a quarterstaff or spear), and is a bit better than Fire Bolt, so it's worth considering.
I really like this because it means we get to feel like a Necromancer right when we get the subclass. And that's minion number 1 who should be by our side at all times (unless it gets killed).
Now, at level 6, we automatically get Animate Dead (hey, take Summon Undead to tide you over at level 5). When we cast it, it's automatically upcast by 1 level. This is actually really great, especially at this level, because Animate Dead is the rare spell that actually upcasts far better than its base version: at its base 3rd level, you just get one minion, but every spell level beyond it adds two, so at this point, you can have a total of four skeletons on your side - one from Find Familiar and three from Animate Dead.
Actually, the math here is interesting. If we were level 11, say, and wanted to maximize the number of minions we could get via the spells regained from Arcane Recovery, do we want a 5th level spell or two 3rd level spells? Because they're upcast, we'd be getting 7 minions from a 5th level slot and 3 from each 3rd level slot. So it's actually more efficient to use the higher-level slot.
Anyway, we get one free cast of Animate Dead per day, which also benefits from this auto-upcasting. Now, the other really great thing is the following:
Each undead we create with a spell (not just Necromancy spells, so our undead familiar will benefit) gets additional max HP equal to our Int plus half our Wizard level. If we assume a +4 to Int at this level, that's 7 extra HP, which is a roughly 50% increase for both Zombies and Skeletons. By level 20, we should expect 15 extra HP, which about doubles it (and notably makes it possible that they'll withstand a Fireball, though a Skeleton with this bonus will be right at the average damage of one).
They'll also get to add your Intelligence modifier on a hit with their attacks. Again, that's something that scales a lot with numbers. At higher levels, you can grant undead creatures you create temporary HP equal to your Wizard level once per day, which will make them more resilient.
Ok, again, let's talk practicalities:
First off, your minions are a bit more resilient, but what do you do if they die?
Well, here's one weird thing that you'll certainly want to check with your DM about: is a dead zombie now just a corpse? And is a dead skeleton now a pile of bones? In other words, can you re-animate a re-killed undead creature?
See, one of the things that the Necromancer always has to worry about in a way that the Conjurer doesn't is that, typically, you need materials to raise if you're a necromancer. That's not always the case - sometimes you summon them forth from the underworld/Shadowfell/wherever (my Wizard has Summon Undead and I always flavor it as if they're coming through a portal to the Shadowfell, usually taking on some idiosyncratic personality).
I think a good thing to discuss with your DM is how available materials should be for you - and I'd advise DMs to make it reasonable for a player to use this central mechanical concept to their character. There can, though, still be some social consequences to having a bunch of undead minions around you all the time, but I'd generally try to find a way to let them have their cool thing.
Now, I think it's possible that you'd want to use Zombies, but generally speaking I think the Skeletons are the superior option unless dealing with foes who do bludgeoning damage.
I'm not going to do a full "vs Wyvern" damage calculation, in part because I want to have the book to really build something out with tons of feats and such, but practically speaking, let's figure out what we could do with a barrage of skeletal archers shooting at a target.
At level 10, we have 5th level spells. Assuming we want to invest in our undead army but not use up all of our spell slots, let's say that we've cast Animate Dead at 5th, 4th, and 3rd level to build our army. That's 3 from the 3rd level slot, 5 from the 4th level, and 7 from the 5th level slot, which means we have 15 skeletons. We have a 16th from our Find Familiar spell. Each has a +5 to hit (with either attack, but it's obviously better for them to stay at range and fire their shortbows - preferably spread out quite wide).
If we have +4 to Int at this level (we might want to pick up some general feats at 4 and 8, so we'd be at 19 probably) then each of these skeletons is going to hit for 1d6+7, or 10.5 on average.
If we're attacking a Wyvern (just to be consistent,) that means we hit on a 9 or higher with our +5 to hit, so that's a 60% hit chance. 10.5x60% is 6.3 damage. And then we add 3.5 5% of the time, giving us an extra .175, so we're looking at 6.475 per attack.
With 15 such attacks, that's 97.125 damage. It's heavy spell slot investment, but it's non-concentration, and unless our foes blow them all up with AoE (which isn't impossible,) that's pretty nutso, given that we won't, I believe, need to spend much if any of our own action economy on it.
But let's consider the downsides:
This is going to be rough on your table. It's going to take a long time to position all of your minions, and rolling every attack and all your damage rolls is going to take a long time. So, let's talk best practices:
First off, we're not rolling damage anymore. Using the average damage options for the stat blocks, we'll say that a hit does a flat 6 before we add the Necrotic damage. We can say that a crit will do a flat 10 (the average damage in the stat block rounds down, but 2d6+3 is straight up 10 on average). This way, we're only rolling d20s and cutting down the rolling and counting.
Next, DMs, tell the player what AC they need to hit. Yes, sometimes it's fun to make a guessing game out of just what you need to hit the target, but this way, a player can simply know what they need to roll and quickly roll 15 d20s and say "ok, that's 8 hits and one crit" and quickly figure out that that's 94 damage.
If we want to go to a real extreme, we could also just have a single attack roll for the entire undead horde. This will make things swingy, to be clear: it becomes all or nothing, but you're looking at a monster either taking zero or like 150 damage (or 210 if it's a crit).
Basically, don't play this class if you're the sort of player who isn't thinking constantly about what you're going to do before the DM gets to your turn. I have some players - whom I love, to be clear - who will take several minutes to decide what they want to do on their turn, usually because they've been distracted and not paying attention to what happened since their previous turn, and some who will have a plan and then if circumstances change (like a monster behaving differently than they expected) will take a long time to adapt.
If you want to play this style of character, you need to be on the ball, and even risk some suboptimal choices in the interest of alacrity. D&D combat can go very slow, especially with a lot of spellcasters in the party. Try not to be the reason that three-round combat took over 5 hours to finish.