Depends. Spending years researching security and finding as many possible attack vectors and accounting for those, plus, building tests for the situations will make for a very hardened system.

More eyes on security, can also mean a short time of eyes by a single person on a single problem - missing the bigger picture. Adding to the problem, security researchers are very few and very busy (I've hired). They don't have time to look at open source code, unless there's a possible reward (that's big enough).