Seth Rogan isn’t a good actor but he has that friendly voice and attitude so people like his performance anyway. Most Seth Rogen roles are just different flavors of himself.
I don’t really like Allen’s performance, but the personalities are similar enough that it’s not unbearably bad.
My dude, I don’t know how to tell you this… But Viltrumites are basically Saiyans with a mustache instead of golden hair.