I would only agree with the above comments in respect of the really cheap end of the hardtail/full suspension bike market. Those things ARE total junk BSO's and will be a choice you'd regret. However, if you stick to bikes with simple, rigid steel frames even cheap ones can be durable and long lasting.
My hack bike came out of a builder's skip, and is an old Apollo MTB from the mid-1990's, which I use very regularly including some light offroad riding. Whilst it is quite heavy compared to an aluminium or carbon fibre bike, it is NOT clunky or poorly made. It is just a cheap bike sold originally for a low price. It can't be that terrible if it's lasted nearly 25 years!
Cycleops commented about Sports Direct, what they do is buy up old, tired brand names from the past, and sell those products at low prices. Ashley is a very rich man, so this strategy must have worked. So long as you are aware the brands like Muddy Fox aren't what they were 25 years ago, and don't expect the same quality, I see no particular reason to avoid other than if you object to their controversial employment practices.
Any old bike can be a gem or can be junk, and junk can often be turned into a gem with a bit of spannering and lubrication. The average cheap new bike probably has just as much wrong with it as the average old bike, so don't assume new equals no hassle, especially at the budget end.