I've had several, been pretty lucky with them. Two Impact, an X upgraded to a Mk2, and a Mk3, a Cyclone, two Royale 400s, a Crown Mk2, maybe some others.
Let's face it, there are two, "almost" imperatives when shooting pcps. One, you're going to have to learn to work on them and two, sooner or later, you will deal with a leak. It's just part of the game. Now I will admit that FX pushes development hard, adn with that there will obviously be issues.
Also, they make what people want, or they wouldn't sell and they would slow down. Second, FX likely outsells all other airguns, so someone is buying them and not complaining.
One problem is for instance, the Impact. It is a spectacular gun, but it is not a gun for the first time pcp owner, not is it for the casual shooter. It is, by design, a tinkerers gun. The Crown is only a slight bit less intimidating to new users. Are they good guns? you bet they are. My Crown usually outshoots anything I've had and easily equals the best.
Designs like the Cyclone and Royale, were stellar, simple, robust and reliable. But, there's a problem, they didn't sell well because that particular niche' in the market is well covered. There are tons of good, simple, reliable pcp rifles.
As to the Amp regs, they're pretty simple devices. The only real problem is if you let the oring inside the action go long term and it gets hard, it's a pain to remove. As long as it's soft, just stick a pin in it and work it out. I use old time hat pins, works great.
To conclude, I think FX fills a particular need in the market for airgunners who want the bleeding edge rifle, even though it may have some growing pains and need to be tinkered with. My MK2 Impact was one of the most reliable rifles I ever had, but many have trouble with them so it seems. Sometimes I wonder if tinkering didn't cause a lot of problems. Dunno?