BFA and Shadowlands were both periods in WoW where I was just not doing much in the way of raiding, even on LFR. The tuning of LFR's difficulty has always been a kind of question: how much to they actually want players to struggle, and how much of it should be essentially the "normal dungeon" level, where the most basic kind of competence is all that is required to succeed?
Personally, I think LFR in particular should be smooth and easy. And I think they kind of nailed the right difficulty in Aberrus and Amirdrassil - it wasn't that you'd never wipe, but it was only if your raid truly ignored the mechanics of a fight.
Anyway, I think that it was just a tad more difficult in BFA, and on top of that I was getting pretty burnt out, hating Azerite Armor, etc.
I wound up only doing enough of the Battle of Dazar'alor to finish the Horde side of the story campaign (which meant basically just running the final wing on my Shaman) and then didn't even set foot in the Eternal Palace. I did come back for Ny'alotha, but for whatever reason never got around to the final wing.
(I'll note that I haven't been doing Nerub'ar Palace as much on LFR, though in this case it's largely because I, frankly, outgear it thanks to Delves.)
Anyway, Shadowlands was an expansion I was more into than BFA, and I did all of Castle Nathria on a couple characters. But it was shortly into Sanctum of Domination that the whole scandal at Blizzard broke, and I found myself less motivated to play while I reckoned with that. So I wound up only doing the first wing of Sanctum and then, like the Eternal Palace, didn't set foot in Zereth Mortis - though this latter was odd, as I did come back for that patch. I think I just felt anxious about stepping into LFR.
The general rule is that you can pretty easily solo raids that are two expansions ago. In the end-of-expansion doldrums of Dragonflight, I finally went back and got through the BFA raids I'd missed (I still think the first part of Ny'alotha really nails the "cyclopean architecture" of cosmic horror). But just today, I finally did what I could of the Shadowlands raids.
There are a couple hiccups in Sanctum of Domination. First, there's a pull before the Eye of the Jailer fight where you need to draw some guys from the opposite side of a big spell effect that knocks you back. Earlier, when I tried to solo either Heroic or Normal on my DK, I was able to use his Venthyr Door of Shadows to cross the gap, but my paladin main had no way to do so.
Luckily, though, I had already done that wing when it was current, and I was doing it all on LFR (which is also convenient for giving you places to pause) so I just skipped ahead. Unfortunately, I then had an issue on Kel'thuzad, who requires you to go into his phylactery to defeat him, but if the main room is empty he despawns. So, I just had to leave him undefeated.
I know this is like three or four years out of date, but boy the Sylvanas fight is bonkers.
Sepulcher of the First Ones I really feel a lot of regret about not doing when it was current. The aesthetic of Zereth Mortis is genuinely unlike anything I've ever seen in WoW. I don't know what it is about it, but I feel like there's a kind of slick, 1960's vibe to it, whether it's the colors or the simple geometry.
Lorewise, of course, the "First Ones" really complicates and convolutes what we know before (muddying the waters by giving us even more ancient and powerful beings than the Titans who also have an oddly technological expression).
I'm really curious to see if and how they'll manage to sort out the cosmic stuff - personally, I love fantasy that gets weird and otherworldly, but I suspect they're going to get more mileage out of the nuances and complications of "hey, you know how earlier the Light and the Titans were generally seen as the ultimate forces of good? The truth might be more complicated" than this whole idea of... kind of meta-Titans.
For instance, it feels like Dragonflight and War Within are both really emphasizing a reckoning with whether the Titans were really all that benevolent in what they did to Azeroth - the Dragons coming to realize that their roles were kind of forced upon them, and the Earthen beginning to realize that the rigidity of their Titan edicts is just not suited to the dynamics of the world they live in.
Mechanically, of course, I can only kind of guess at how these fights would work - I liked the Fate-Scribe one in Sanctum where you had to align a bunch of code wheels. And I had heard about the one in Sepulcher that you have to chase across a huge map, which I somehow expected to be faster movement and a lot of starting and stopping, rather than the continual thing it was (also, I got it down about halfway down its path - on a tank spec).
I think it's a shame that Shadowlands really suffered from a combination of Covid, Blizzard's big scandal, and perhaps a bit of convolution to its story (which might have been the fault of one of the prominent people at Blizzard whose actions were among the most atrocious).
I sure hope that the people working at that company are doing better now - the scandal has kind of receded into the background, especially given the success (at least as I perceive it) of Dragonflight and what looks like it will be a well-received follow-up (have we ever had two expansions people almost universally liked in a row? Maybe Burning Crusade and Wrath? Though I don't know if BC has aged as well in peoples' minds - of course, it's early to judge War Within, but it's definitely got a strong first patch).
Basically, the concept of Shadowlands - a peek into another realm of existence, and giving the forces of death equal footing with other cosmic forces like the Void and the Fel - was something I was very into, and there's a ton of stuff to really recommend that expansion, but I don't think anyone could argue it wasn't flawed in many ways.
But it's cool to go back and visit, and to see things I missed back then.