A friend of mine once (march 2004) bought one of those nasty cheap bikes, abeit a pre-assembled one. I had the "pleasure" of collecting it for him.
Within 50 yards I noticed that the entire thing hadn't been assembled at all. The levers were loose, the headset was dry and loose, the stem wasn't tightened to the fork and the gears weren't running properly.
Upon noticing that my input at the handlebar had no influence whatsoever on the front wheel's position, I tried to brake, only to feel the laft front brake pad missing the rim completely and ending up between the spokes. Luckily I was doing 3 MPH.
I managed to make it home and went over the entire bike, tightening every single bolt and nut and properly tensioning the spokes.
I then delivered it to him. I was kinda worreid because he's a rather overweight guy.
The first time he got on it, the seam on the rear inner tyre gave up under his 250lbs, and the outer tyre got severely damaged due to the resulting snakebite. A new set of outer and inner tyres solved that.
The next day he took it for its first proper ride (on the road), and managed to bend the front rim because he had to brake mid-corner. Decathlon refused to provide warranty because we had made modifications to the bike by replacing the tyres, so we bought a set of cheap wheels and all was good again for a while.
2 weeks later I took his bike out for a test drive to adjust the gears, and noticed that the frame itself was flexing. I adjusted the gears and told him that I'd like to ride it again after 2 more weeks.
On my second run I noticed that the flexing had gotten worse, up to the point where sporty riding would make it go into a high-speed shimmy which reminded me more of the sort of tankslappers I was used to on my old motorcycle. I also managed to break the chain.
I fond no cracks, so the bike was then taken back to Decathlon. Without even trying it, they told me that the flexing was all in my head. Meanwhile the fork was starting to seize and the chain and sprockets were showing signs of wear ( after less than 500 miles ! ).
We decided that it would cost more to reinforce the frame and fix the bike up than I paid for my then Bulls Sport 3.50, so he ended up buying the same one I had. He still rides that Bulls.