I shoot STI's... a lot! In the 100,000 to 200,000 range in the last decade. There have been some quality issues with a select few models of STI's that the company stands behind and replaced the entire gun for. My round count is obviously high, and yet they replaced the chipped slide on one of my guns at no charge (gunsmithing and shipping included). There are other guns that are as good as STI but STI's warranty and service are not going to be surpassed.
Without knowing the exact situation of the shooters with "jam-o-matic" STI's, my first inclination would be to look at the ammo, next the mags, and finally the gun itself.
If someone wants to sell their "jam-o-matic" STI's cheap, there will be a lineup of people jumping on the opportunity to buy them.
-ivan-