For one, all of their internals are made of high quality metals (not the cheap pot metal WE uses) so they have much fewer issues with breakage, you can increase the weight of the bolts extensively and increase gas flow even more, resulting in fantastic recoil, you also use Pro Win mags, which are much better than anything from WE (mine hold gas for several months at a time, and I rarely remember to lube them). If you want further proof, go to
http://www.gasguns.info/forum/index.php Inokatsu and Viper tech GBBR's are king, I've owned WE's and they can't hold a candle to the utter stability and strength of my Inokatsu; forged aluminium receiver + all steel internals + 50 round magazines that never leak = goodbye WE.