Nere: I hate the way he Thanos blips your allies with his legendary action. The way that forces changes in how and when I feel safe attacking him is annoying.
Balthazar: You get a necrotic AoE, and you and you! The endless resurrections, his mobility, and even worse yet if you stick to siding with him you get a far more difficult battle that shuts down your dedicated spellcasters at the start of it.
Gondians: "The quickest route to freedom is exploding an oil barrel in your own face. Or standing idle in front of ladders. Gond HATES when his worshippers have a sense of self preservation." I feel like I spend the majority of the Iron Throne encounter skipping my turns, waiting on them.
Oliver: Necrotic AoEs are great, aren't they? That whole fight feels specifically like it's made to punish melee users.
Orin: In a change of pace, I have been getting a glitch for about a year that causes her not to attack Durge while in Slayer form. She'll do Sanguine Lash as a reaction, but she won't spend actions on attacking. This makes the fight literally so easy that I can't lose unless I actively try to. I don't remember the last time I had a non-glitched Durge/Slayer Orin battle.