here is my take: yes, diversity is overpowered. this is a game about space empires, maximizing our yield on all planets in your space is sort of how you do it
but adding races to your empire isnt the only way, just the most dangerous. you could opt for some simple, nonsentient robotics, at a cost of 2 energy per, the maintenence is steep, but you dont have to worry about rebellion
you could also terraform planets or genetically engineer your people.
all these choices come with some inherent risk, but by far the riskiest and cheapest is being the space austro-hungarian empire. conquer weaker neighbors, uplift as many races as possible, and have open borders will all equivalent empires in your space. if this sounds like an awesome idea, try playing as them in vicky 2, youll understand why it isnt. you should, if you are playing xenophillic, have alien pops in your empire, but even if your strategy revolves around xenophillia, youll have to be somewhat selective. what do you think happens when you conquer a fanatic pacifist race that lives on literally the exact opposite planet type as you? you think they are going to be able to integrate at all? that their ethos will change, living in a 100% homogeneous society with none of your primary pops? probably not. same with conquering repugnant fast breeders. ew, get that away from me. these races are better suited as slaves.
youll want to be conquering, uplifting, and accepting the migrants of races that have similar homeworlds or ethos(one or the other) to your other, accepted pops, so they can integrate sucessfully. otherwise, youll have issues.