Not always true.. I've ran Tusk kits in every one of my bikes.. 2 Banshee's, Raptor, YFZ450 and my 250r. Everyone of the motors was built and they held up... Oh.. and they also held on my 70+ hp drag bike with no lockup. (and thats taking off your so called 15%)
I ran a Tusk kit on my 4mil drag ported cub with no problems for 2yrs.. And i'll probably be running the same Tusk kit next year. Tusk isn't a bad kit and is a viable option. But if you sleep better at night just by spending 60-70 bucks more on a clutch kit. More power to ya.