My personal experience was bad with a Walmart bike, specifically a Next. Cost me literally 70 dollars and was as happy as a clam. First ride was something like 10 miles, everything went good. The ride was on road, never even dawned on me back then to ride off road with it. Every ride after that my non drive side crank arm would fall off. Started getting some ghost shifting for no real reason. After it sat around for a few years, my roommate and now best friend/biking buddy got me into wanting to ride. So I took my bike off road, which was a bad idea. Ghost shifting is horribly frustrating when riding a trail. My crank arm fell off and we were unable to find the nut that holds it on. Ended up using one of the nuts the held the front wheel on. So I literally road home with one nut holding my front wheel on. My back wheel was so out of tru that I had to disconnect my rear brake just so I could peddle.
That was my last experience on that bike. My room mates brother donated to me a old Rigid Steel Schwinn Single speed, until I started to build my first bike.