When I export FBX with Unreal preset and choose to merge all materials, CC3 decides to randomly split some UVs into small pieces.

This makes it very hard to predict the outcome of the exported texture and makes it nearly impossible to edit the textures after export.

Some kind of preview option for the merged UV would be a huge boost the workflow and toolset.
I'll also echo the prior wishes of smarter UV optimization based on object size to get better texel density.