I've heard a lot of this talk on a lot of forums.
There's a reason behind it. And most of the time that reason is fear... Fear that people could actually buy a cheap bike that's good enough to sustain whatever they need, and that they will not receive sales through their own channel.
The bike store too, telling me how bad these kits are for the bicycles, showing how the sprocket killed the spokes.
Then I think to myself, EZ remedy! Get a smaller sprocket, that means less torque on the spokes, accelerate a bit more careful, and make sure that the spokes are all tensioned correctly, and these things will last just fine!
In every category, there's going to be an idiot, trying to "tune" the thing to make it go like a rocket, and in the process overburden some parts.
Same on scooters, and then they wonder why the belts break...
I've heard a lot of 'warnings' from people saying the same thing about chinese scoots (they usually work in Vespa or other brand stores).
Well let me tell you,
my $800 Chinese scooter, has over 2500 trouble free miles on them, and more to go!
Worth buying?
Every penny of it!
What would I have gained if I had purchased a Vespa?
Probably less minor adjustments where necessary, fewer oil changes; and perhaps the comfort level was a bit better.
But I would have lost over 3-4x the amount of money as on my chinese scoot, and the vespa's aren't even performing as good as my china bike.
So moral of the story:
There are always nay-sayers on every forum.
I'm sure some of your fears are grounded on the weakest of bicycles, but you're not going to tell me that a cruiser bike's frame is not supporting the engine.
In my early years, I was an avid bicycle rider. I rode with average speeds of 20-25MPH (usually did very short trips, less than 20 minutes, but also did occasional 1,5 hour rides)
In the short trips, my speed often surpassed 30 mph (esp with wind in the back).
In the long trips I rode less agressive.
If there was anyone that torqued the frame, it was me. And on one of my bikes I did manage to break the frame on the pedals, because of hard acceleration, causing the frame to wobble when riding without hands.
But that bike was nowhere comparable to the Mountain and cruiser bikes I see in Walmart today.
Even I think I'll have a hard time bending those frames!
But then again, the way I plan on using the bikes is totally different from most people
Most just want fast acceleration.
I want low RPM, and great gas mileage, and will be using the engine in the lower end of the rev range.
And again, to be able to do 40-45MPH doesn't mean I'm always going to ride at those speeds.
I just set up the bike like this, to lower RPM, and increase MPG, and lower engine wear in the process.
On top of that, the Genesis Onyx bikes are hardly to be called 'weak bikes'.
I mean, overall I agree with your statement that there is danger, and there are bikes in WM that are very fragile.
But not all of them are (take a look at the fat beach cruiser they're selling now. I don't think any bike in the bicycle store (even with a steel frame), can topple that one in terms of rigidness.