I think I just good morning sickness. AP was always the issue, as far back as Legion were you couldn't play more than one spec at the start (especially if you got the wrong legendary on top), and I'm saying this as a person who usually prefers to play one spec per character anyways. Essences were also the biggest bullshit ever, on par with their conduit cousins from Shadowlands. On top of that, all of the shit you had to farm in order to play a patch were gone as soon as the next Expansion hit .... if they had build on artifacts in Legion the whole system could have worked, but making everything irrelevant after an expansion is literally the antithesis to WoW. What made this game big is that whatever you invested stayed for your character for ever, which only got solidified when WotLK prepatch hit thanks to achievements, and even more so with transmog and other collections.
Even corruptions were only cool as long as there was no vendor (yes, hang me) since as soon as you could choose which corruptions to use you were stuck with your "bis" corruptions, so it was all just a grind with an illusion of choice anyways. Twillight Dev was fun as a tank though as long as you didn't had to get versa ones because your key level got over 21.