Not really, although I'm a not that a techie myself. It depends how many people using the software, you're not going to find that much hidden bugs. Like phpBB, where more than 66,000+ websites using it, one can see something somewhere... When a "free" software becomes so popular like PhpBB, some jealous people wanted to exploit it.
I think commercial is better in terms of support-wise. You may get faster replies. Correct me if I'm wrong, One time, I did seek help on the PhpBB support forum, it takes days instead of hours for someone to reply on my topic - really.
Bottomline, there are always pros and cons between Commercial and Open-source. And the web users have the opportunity to balance them.