FWIW ... I recently got a Serpa CQC for my Gen4 Glock 23, so I haven't had it long but I've used it a bit. My basic thoughts:
From what I can tell (following what RedRuger mentioned) some people fail to press on the retention mechanism properly and they occasionally end up with a negligent discharge. This happens because they curve their finger to get more pressure on the retention mechanism, and then the finger may slide inside the trigger guard (and possibly press against the trigger and fire the weapon). Using it properly - with the finger kept straight - should prevent this problem.
The retention system seems to work very well. Out hiking it should be plenty strong enough to keep it in place. One concern that comes to mind - and this is my speculation, not based on any reports that I have heard - is that if you're open carrying, anyone who knows how to release a Serpa holster might be able to make a decent attempt to pull the gun out, since the retention mechanism is released on the outside of the holster.
This could probably be stopped by having a forward cant, using situational awareness, being prepared to defend against an attempted theft, etc. I don't know if this type of attempted theft occurs much, if at all ... but it might be more of a concern vs. a holster that has a retention mechanism that is disabled closer to the body and/or on the inside of the holster.
I don't know if it causes finish problems or not (I haven't used it long enough to know yet).
Hope this helps.