I have used Umarexes for IPAS mainly a tuned CP88 and never had a jam. In fact I own 4 CP88's and they function perfectly. Mind you all of my these have been the older models which I believe had better build quality.
The only jams Ive ever encountered is when I was new to this type of pistol and hadnt seated the pellets sufficiently in the 8 shot magazine. I use a small piece of hardwood dowel rounded at one end to do this now - Ive seen others use various implements. I always use RWS Hobbys. If you watch shooters at IPAS matched reloading you will see them all taking care re seating pellets.
Hope this helps.