FOX engines are by far the worst engines I have ever dealt with. I bought 2 planes on RCU that came with them and eventually had to rip both of them out. The engines would stop running at the worst opportune times (such as during rotation or over the woods). Nobody at the field could tune away the problems and the fuel tank was set up properly too, so it must have been the engines.
In terms of reliability and durability of all the engines I've owned, it has to be 1) West, 2) OS, 3) Rossi and 4) MVVS at the top, then 5) Jett (sorry Jett-fanatics, my Jett .50 has started to poop out on me
) , then 6) Magnum and 7) GMS in the lower middle and finally 8) FOX