Majority of bosses got ridiculously buffed and there were two sides of the conflict: those that liked it and wanted them to stay like that and those that didn’t like it and thought that they were unfair. I think that both of the sides had some good points, but for me personally U.C.Os were a little unfair.
tl;dr
bosses got unreasonably hard (IA words not mine so shush)
Here are related topics if someone wants to do their research:
Early Access version 10
Early Access version 10.3
Pease Nerf and Fix
The Yolk-Star™ with maximum difficulty
I remember there was also another topic (probably much later), but I can’t find it.