One million percent reliable. We've used them for WDW and for Uni.
Over on the WDW side, there have been some people who have trouble at WDW, but once UCT is called and gets involved, the problems go away, the Cast Members are taught the right way to do things, and everything ends up better. (and I can only recall two times since I joined that there have been issues). UCT is totally authorized, they are known to WDW (and Uni), and they are terrific.
And no I don't own them. I haven't even used them in years (we keep APs for Universal). But they are great.