It's fairly easy to screw up an aftermarket mag (given my experience w/
various types over the years). In the case of these 30 rd'ers, I had a
friend who bought a couple for a real HK91 and neither fed right. No a
big sample size, but a small real sample.
Aftermarket mag problems off the top of my head: angled wrong usually
due to mag body being too small or other dimensions being completely off
from the start, notch cut in wrong place, notch cut too big/too small, soft
feed lips that spread or snap off, spring way too weak/strong to feed the
rds properly.
My absolute favorites are the handgun mags which are too fat to actually
slide into the grip or too short and the bottom plate keeps it from going all
the way up and locking in.
And let's not forget the potential for mag follower problems....