The three that come to mind right away are Gravewalker, He-Who-Walks-Below and the Arena Champion. Probably Thag, too, but I’ve only fought him once and that in company of another player with his überthrall. I haven’t been to the Sunken City in a long time, so I’m not sure about that anymore.
Of the three I mention above, the Arena Champion is the one that hit the hardest. Sure, you can stun-lock her and you can bleed and poison her, but if you mistime things just once (or if the latency gods decide to punish you), you’re very likely to die in a couple of hits and have to go through the whole rigmarole of getting spare keys, running to your corpse and getting your stuff, all with Yakety Sax playing in the background.
The Gravewalker and He-Who-Walks-Below don’t hit as hard, but they still hit quite hard and they present other difficulties. They’re both undead, so you can’t stack DoT effects on them without fiddling with spectral coating. On top of it, they both stack corruption on you.
None of the bosses I mentioned is impossible to defeat without an überthrall. If we’re framing this discussion in terms of “being a hero”, then it’s not enough to just consider whether defeating certain bosses is possible or not. It’s about how you feel when you play the game.
All bosses hit hard and have lots of health, but the degrees differ. There’s a certain fuzzy threshold where the quantitative differences become qualitative. This threshold is somewhat subjective, so it’s hard to argue about this objectively.
That said, I don’t think we can objectively deny that Funcom seems to be balancing certain aspects of PVE content around thralls. To me it seems like they’re in a vicious cycle of making thralls stronger for one reason, then introducing PVE content balanced around these stronger thralls, then making thralls stronger again, etc.
I honestly don’t feel like a hero running through the Undead City and the Warmaker’s Sanctuary with a thrall doing all my dirty work. I feel like a sidekick.
Neither do I feel like a hero running through these places without a thrall. The way I’m forced to fight without a thrall in these places kills the immersion.
I will happily fight the Red Mother or Rotbranch or Ladagara without a thrall and I’ll feel like a hero, because it feels like a battle. It feels like a contest of strength, dexterity and cunning, despite the limited AI. With undead bosses and the Arena Champion it feels more like a rat nipping at a cheese in a mousetrap or a lumberjack cutting down a tree with a butter knife, depending on the situation.
Whenever I see an argument along the lines of “if you think it’s OP, just don’t use it”, I always go “that’s not how any of this works” 
I think the same applies to the thralls. Of course, that doesn’t mean they should just nerf thralls and leave things at that. I would love it if we could get a future patch that balances Conan Exiles in such a way that the primary game loop isn’t so full of tedious chores, but I realize that it’s a lot of work and that I might never get that wish fulfilled. But I do understand and empathize with arguments against the überthralls 
PS: My apologies if this post is even longer and more rambling than my usual. My optometrist screwed up my prescription glasses so I’m sitting here with everything blurry and trying to write despite that. I can’t wait to get home and put my old glasses on…
!
