The game was at its core completely demolished
When servers went back up after implementing CE2, we didn't have teams, taming or many other features we consider standard today.
Also, we still havn't been able to sit or lie down on the ground since. =(
It took many years before taming was introduced again.
My point is.. when the engine was changed there were massive changes in the game. A lot of features wasn't ready when engines was swapped.
I don't think that will happen now though. I think MA will make sure everything works with this move, since there are so many more developers ready to work with UE