Never had a malfunction with the colt/uzi pattern mags other that if you drop a full one it might puke up some rounds.
Biggest problem for me was reloading and esp on a classifier. I just seem to fumble the Colt/Uzi mags as opposed to glock mags.
I also find it is harder to insert a colt/uzi mag on a closed bolt as opposed to a glock mag, esp when it is near max capacity for the mag.
Both of my lowers are QC-10.