Well, symptoms of problems tend to be more evident than their root cause, so commonality of claims is not a particularly good reason to rule out nepotism being a problem.

Evidence would be nice to have though.

As for me, I've had enough anecdotes to think it's plausible. Just recently we hired a new CTO at my company, and he got the job simply because he was friends with our CEO and he needed a job. Turns out he's completely incompetent...