Being hard isn't a weakness, because it can be fixed by being good at the champion. Skillcap is a balance problem more than a balance helper. Example: Champions such as azir will most likely be better than ones such as annie because a good azir can do so much more than a good annie unless azir is thrown into the garbage can (right now)
It also creates nearly powercreep situations, as Yasuo versus tryndamere for example. you simply can do more and will be more useful to your team on yasuo than tryndamere, and the only real trade off is early game RNG and sustain.
Vulnerability to cc
Lack of CC
Lack of engage (mainly for melees, but also applies to ranged too)