The number of times a lucky sleep all gets 2+ monsters asleep and it’s instant GG… most frustrating thing to happen in a match. That’s the main complaint, but also if you have the wrong monsters then it’s a huge pain to get rid of and all the while you can’t properly touch the rest of the team.
Ziberius. The monster is by no means unbalanced or OP, quite the contrary, but he thinks he’s so cool by killing his allies and charge his own Retribution… Hence the nickname Frosty McShitface.
Many other monsters do so too, but he was the first one (heh) to do that. Plus, at least Angelion brings a soul back, and Hellfox doesn’t actually kill anybody for real with Rebirth.