I have a Del-Ton mid-length kit on a DPMS stripped lower. I was having multiple problems with cycling, feeding, FTE's and stovepipes. I sent the upper back and they determined it to be something with the bolt. They received my upper, test fired it, stripped it, spec'd it, repaired it (replaced the bolt assembly), cleaned it and shipped it the same day.
When I got it back I ran 200 rounds through it and it ran flawlessly. You can have problems with anything mechanical. It's the customer service that separates companies.
BTW-mine is a tack driver with A2 fixed sights.