The lair bosses particularly are just a horrible mismatch with the rest of the game. The vast bulk of the game is basically a headlong rampage across the land and through dungeons with some tougher enemies now and then, and then suddenly the lair bosses are all fights about carefully dodging telegraphed attacks and dancing around dodging one-shot mechanics within tiny, constrained rooms like some sort of bloody WoW raid for one! They're obnoxious, and I say that have defeated all of them on Torment 4 (the max difficult) countless times.
Andariel, Harbinger and to a lesser extent Duriel are particularly annoying, because they're extremely lengthy fights were the actual boss is attackable for maybe 10-20% of the fight at most, and the rest of the time you're just "doing mechanics". With Harbinger your build/gear is barely even relevant, you're just there to pick up orbs that Akarat drops and then shoot them. With Andariel you barely get to attack her before another immunity phase starts and you have to blow up three statues which have like between them like 2x as much HP as she does! And you have to do that up to four times! Just dodging annoying instant-death beams the whole time. At least with the basic bosses you can potentially just overwhelm them.
It doesn't help that a fair number of builds that are pretty effectively generally just don't work well on bosses either (but the same doesn't generally apply in reverse).
D4 has great visual and sound design, and some good ideas, but it's basically got the mechanics of a slightly confused game that's deep in Early Access. Literally PoE2, which is genuinely deep in Early Access has a more consistent and together visions of what its doing, mechanically (which includes boss design).