I think the Supermarket BSO has several faults. They are designed for purchase by non cyclists to look good in the shop and have whatever 'features' are cool with the kids at the moment. So you get a full suspension MTB for £80. To add insult to injury it is then 'assembled' by a shop assistant who frequently has no real knowledge or the skill to do this correctly.
When I returned to cycling in 1999 I bought a cheap bike from an LBS, Cost was around £150 in a sale. The bike was a Scott Atacama, and as new I found I could pull the rear wheel skew in the frame under acceleration. The LBS replaced the QR with an axle and the problem was solved, they did this at the free 6 weeks service, along with truing the wheels. I sold it a few years later for £50. The bike was basic, 21 speed and cheap fittings and accessories, but it commuted to work, took me on day rides and did everything a bike should do. I replaced it with a Thorn Club Tour, at £900+, which had better components and did everything a bike should do. Was it worth six times my Scott? To me yes, but I could have happily continued with my old bike if money had been short. Would a Supermarket special do what either of these bikes could do? I don't think so.