Many of their previous updates on the unity engine broke mods - for something as fundamentally massive as changing the entire game engine, it's an incredible achievement to have done so while preserving the functionality of the tens of thousands of 3rd party scripts.

I mean, it seems simple - just make the new engine compatible with the old mods....

but changing engines is hard work. building your own engine is exceptionally harder. lots of things to break.

3rd party mods are often not supported and may rely on quirky unity behavior. I don't know how much of unity they used but that sounds very impressive to me that they managed to keep mods working as supposed to all mods needing a minor update.