For the most part, nothing. Both, especially Fox, have have varing degrees of quality control issues, that is going from good or great to poor and back again, but I think both are good right now. Also both especially Fox have had models with tuning quirks. That is they would run fine, but you had to tune by their insructions, not the OS way.

I think the more modern American engines, namely Jett and Nelson have high regard in both quality and performance. Unfortunately their price reflects this high regard.