No in-frame kit is truly "bolt n'go" and I suspect that misconception is where a lot of the "unreliable" complaints stem from. While it's true that the low-priced Chinese engines lack quality control, purchasing from a reputable vendor will help should you just get a defective kit - almost all of the rest of the complaints I've seen really do amount to operator error in some form or another, most commonly installation problems & oversights.
Which is where this forum comes into play
Don't get me wrong, I'm not "blaming" anyone, although these kits are simple and the build can
be easy - modifying anything
is an experimentation process prone to problems and this is only more true for the first build. Add in the propensity for folks to "scrimp" on their first build, using the least expensive bike they can find, only the kit supplied parts, and even lacking basic engine experience - all in all I think it's a testament FOR the reliability of these small engines that we've as many successful builds and happy riders as we do.
Heck - I'd blame the kit supplied "rag joint" rear sprocket for at least 75% of the major issues people have had and that's an easy fix... if you're aware that's the root of the problem in the first place