I think it is right to say that most issues are user error or easy enough for them to fix themselves.
Most issues can be fixed with an understanding of a marker, replacing O-Ring or just servicing it yourself.
Yes, there can be issues with the manufacturing process - but usually those issues are picked up. I've seen GT's with metal shavings in the regs. Not great, just breakdown and service your marker when you get it.
I am still learning alot about the Demon - hey, only had it about 3 weeks - but in that time, I blew about 6 o-rings and a diaphragm. Whilst trying to fix the problem, I pretty much taught myself how to fully strip, troubleshoot and maintain the marker. I never had to send it off to Vanguard to be fixed (even though, when it happened I couldn't figure out what was wrong and natural response is to send it back!).
I've known several people with Demons and all love their markers. They hate the diaphragms though
For me, working on the Demon is very similar to a Clone VX... so wasn't too hard to figure things out.
I am still learning, but pretty sure I can fix my marker if a problem came up again!