I've got a gen5 Glock with a belt magazine. I haven't experienced jams, but when I got it the trigger pull was so heavy it was almost unshootable. I lubed it with both moly grease and silicone spray lube and that made a major difference. Shooting about 500 shots through it also helped. The trigger is still awful, but much, much better than when I got it.
In general, I agree with marflow 777 belt fed magazines are a flawed design. There's just too much friction present in them for the trigger mechanism to overcome. Maybe if they were made of a different material and had roller bearings or something they might work better. I notice that some of the UK versions of these pistols use little cylinders instead of belts. That would limit the capacity significantly, but I imagine it also improves the trigger pull greatly.