on the fender mounts breaking, lots of people go to walmart and buy a throw away bike, then wonder why stuff breaks, the difference in the strength of brackets between a decent fender set and the ones that come on those is very nociticable having worked on both.
the main problem with these kits is that they make installation sound far easier than it actually is, if you don't know what you are doing you will most likely have problems when setting up. my first bike the loose ball bearing (behind the bucking bar) fell out without me being aware of it, 14 hours wasted trying to figure out why my clutch would not disengage. this wasn't the kits fault it was mine. third build i over tightened the motor mounts, they snapped, not the kits fault, it was mine. when installed correctly they are pretty reliable, its all upon the installer, remember, we are turning bicycles in to motorcycles, it does require some skill and knowlegde.
here is my two cents, if you plan on buying the cheapest bike from walmart then plan on ditching alot of parts, especially the wheels.
if you upgrade the wheels you can use the stock rag joint, but a hub adapter is still the way to go, a stock rag joint will destroy the spokes on a cheap wheel, some people have gotten away with this, some people have also gone over niagra falls in a barrel and lived, that doesn't mean you should.
use half links to avoid the chain tensioner, if you do use the chain tensioner pack it with quicksteel to keep it in place, if you design a piece like this to fit every bike, it won't fit any bike well. the problem is the extra space between the frame and tensioner allows rotation, fill this with quicksteel and you wont have problems.
be careful not to overtighten the motor mounts, and remember chains stretch and the mounts tend to settle a bit, upon completion take ONE ride around the block and go back over the motor mounts and chain tension. checking EVERY time you ride it for a bit, the mounts usually only take one adjustment the chain will stretch for awhile. if you notice things are loose you can always go back and retighten them, on the other hand if you overtighten and bow the studs out, they are now weakened and you cant go back.
the sparkplug caps are cheap, real cheap, probably the only thing i really take issue with, use them until they break then replace with ngk cap, note, don't remove cap from plug unless you have to and they will last longer.
listen to your bike! very seldom do the bikes just break, they usually give warning first, a sound a vibration etc, listen to it, it will usually give you a heads up.
number one, unless you are a stubborn sob like myself, get on the forum and read. ask some questions, i enjoy the hard way, that doesn't mean you will.