Difference between cheap and expensive mountain bikes
These bikes won't really work well as mountain bikes.

Namely:
They won't be able to handle any real terrain. Will be difficult to control on the trail due to bad damping and insufficient braking or braking modulation. Will be slow uphil due to heavy weight. They gearing will give you fewer options and shifting will not be as accurate. To achieve some reasonable gear range they must use a triple crankset and that is not used for midrange bikes any more (more awkward and heavier.

They will be prone to braking after jumps - I would especially be worried about the wheels (+ axles) and the forks, the steel frames might be sturdy. 

They might work well for commuting, but then some of the features will be just a useless weight addon. Commuters are usually better with simpler bikes from different categories. And not just commuters, even when riding off-road on gravel roads or reasonable paths a simple bike with rigid fork and good rim brakes for the same price might be a better choice with better overall quality.