I agree that it's poorly balanced and mixed groups tend not to go well.
I disagree that the space marine will always outperform a regular human, however. In combat, all a human needs to kill a marine is a las cannon and a high enough initiative modifier to shoot first. These are RP specific situations, though, and do not address the underlaying problem that, unless you specifically skill to kill marines, you're always going to come up short in mixed groups.
Marine-only campaigns can be interesting for those who like marines a lot, I suppose. It's good that the core marines are in the book, at least. The real problem is, as said above, the advanced archtypes, which are severely overpowered and, at times, incorrectly aligned, especially in tome of excess. Things got a bit out of hand there.