I think that there have been a few 'rouge' chips both from Xtender and Enigmah, but its the same with ANY electronic device, theres always a certain % of faulty goods.
The only difference with a mod chip is that because they are mainly being DIY fitted you can never quite be sure if its your chip or your installation until you get another chip and it works
Many people have fitted both types with good results. I personally have been given machines to diagnose that have had chips fitted that have worked for hours/weeks and then just 'stopped' but these seem to be in the minority.
Both chips currently shipping have the same 'problem' with ejecting so even that doesnt separte them.
Personally I think that they are both as good/bad as each other and choice is normally based on price and availability.