If you think that someone could get your gun from you too easily, that isn't a holster problem, that's a training issue. There is no empirical research that shows that a retention holster reduces the probability of being shot with your own gun. On the other hand, anyone who has run cops through qualifications can tell you case after case of cops not being able to efficiently draw the gun, particularly when stress is applied.. If you are willing to put in hundreds of reps with the rig so that drawing the pistol is second nature, great, but the average cop won't, and the middle of a gunfight isn't the place to learn. The reps put into learning a particular holster go out the window as soon as the agency or you decide to change to the latest and greatest, whereas the same reps put into weapons retention techniques work anytime, anywhere, with anything. It also carries over to your off-duty rig.
For the record, when in uniform I wear a simple Gould and Goodrich Border Patrol style holster with a simple thumb break. My plainclothes and off duty rigs are open-topped.
The SERPA mechanism has to be kept super-clean, particularly in dusty, sandy environments, as any grit in the "lock" can effectively lock up the holster. This was learned the hard way, and you can guess where. There have also been a number of negligent discharges in which guys put pressure on the release with their trigger finger and maintained that pressure as the pistol was being drawn, end result being the trigger finger whacking into the trigger as the gun clears the holster.