We've had about 4 NDs into the leg with Serpas around here. Also, I have seen both newbies and experienced officers have to fight the holster for a draw at times.
I have never had an ND with my Glock 19 and Serpa holster, but I have heard of them (and I am pretty sure they are banned from 3-gun use). When they are used properly, there are no problems. If the operator depresses the lock button and doesn't move his finger on the draw, it will
be indexed on the frame, not on the trigger. Also, the only time the gun gets hung up is when you try to draw the firearm before completely depressing the lock.
My Serpa has even prevented a gun grab. However, its nature does make it more prone to NDs; and as a result I would recommend a Safariland ALS as a good OWB retention holster.