Nothing... People who post about them being horrible are either fan boys of Planet Eclipse or don' take care of their stuff. I loved my FX but am currently selling it on Ebay because I came upon a Rev i.
The gun stock is decent, won't lie and say it's amazing, but I had gotten the Techt Upgrades for mine (LAW RAM and Hush Bolt Pro), which changes the gun dramatically. Stock the gun is probably a the worse pick over an Etek 3/4, but with the upgrades shoots as smooth and quieter than an Ego 11.
Don't know JUGGERNAUT's issue above but I have chopped maybe 4-5 balls from the 6-7 cases I put through the gun, and those were because of me underboring too much... Regardless of what you decide to get the FX is an awesome gun when it has the Techt Upgrades but is still good stock should you choose to go that route.