I haven't heard about the overwhelming amount of ND's with the SERPA, but I have read numerous times about how a foreign object can find its way into the mechanism, preventing the firearm to be withdrawn, even under tremendous effort.
We have the SERPA and a Safariland bale holster as available options for service carry.
I didn't experience it for myself, but my line of work involves me potentially getting very dirty, messy, in remote areas with a lot of sand, dirt, etc. and for that reason, I don't want to do a real life field test with the SERPA holster.
"You are fighting for what you can never obtain, and we defending what we mean never to part with." Thomas Paine