Some of the writers seem to follow a policy that only adversary races can be well-rounded in Star Trek. Romulans and Cardassians are still well-rounded. Klingons were well-rounded in TOS, when they were adversaries, but for TNG they became allies, and relatively one-dimensional.
This is how they keep humans "special", and seemingly the logical leaders of the Federation. We're the only well-rounded race in the Federation.
"We don't know what to do about Humans... You have the arrogance of Andorians, the stubborn pride of Tellarites, one moment you're as driven by your emotions as Klingons, and the next you confound us by suddenly embracing logic.
" - Ambassador Soval of Vulcan
Better hope the Cardassians never join the Federation, or we can prepare to see a lot of their complexity vanish overnight.