there's also no reason, other than annoyance and relearning skill rotations, that'd prevent any person/group to (re)clear the 4.0 ultimates while running as GNB/DNC.

I'm willing to bet there'll be a good number of folks that will do precisely that to prove it can be done. Only then for them to say "I cleared UCoB / UwU as (GNB/DNC), but I won't get a weapon as a reward for it" will feel really bad for those that do.