My experience as well...with X39. Haven't tried them with anything else. You have to weed through them and I've experienced about a 1/3rd failure rate. If they work, they tend to just work.It depends on the particular one and the cartridge you're feeding. I had to return some from my initial purchase, but all the ones that I have open now run 100% in 7.62x39 in multiple gun designs. They're even reliable on full auto in my M16.
I think roflcopters was the one that tried 6.5G in them and said they didn't work.