Can't edit my original post, but here's a decent review of a crappy Walmart bike by a dedicated rider.

https://www.youtube.com/watch?v=m3vI2bYhCrk

For the record, I'm probably a slow beginner leisure rider on beginner trails, I probably wouldn't be doing even half of the things in the video so I wouldn't be very alarmed.

He has some other tests of the bikes and what kind of realistic destruction these kinds of bikes can endure.