Are those cases fully compatible with the C64U motherboard, keyboard, mounts, etc.?
Indeed. Commodore sources the case from Icomp which uses the original tooling from the 80s.
I bought one years ago to replace my disintegrating original bread bin case before RF started making their own reproductions of them ( which Commodore also uses ).
I still much prefer the bread bin, there is much more space for you to rest your palms on whilst typing.