I've been doing more investigating. It looks like in the unit builder, you were right, unlimited traits could be made.
However, what is restricted are the champion traits. You cannot assign champion traits to units and expect them to be built, even if you modded them to be henchmen.
Basically if it has the tag:
You cannot build it in the unit designer, with the notable exceptions of the "path" abilities.
Quoting UnleashedElf, reply 601
Actually, is there a list of all attributes for game modifiers somewhere?
No, but this might help you out (Post 1 + Replies 1, 2)
Which game modifier attribute gives the ability for your to give a a perk to a unit?