I have a cheap Halfords mountain bike, very heavy, suspension front and rear which you can't lock out. For my photography it does what I want, if has disk brakes which are very good. Hitting pot holes does not cause problems and in North Wales we seem to collect them. But within a couple of weeks the front cogs had warped and required renewing FOC and the top speed is low, as to being a mountain bike it is that heavy I often end up walking beside it any lower geared one can't balance as too slow, but does not need to be very steep before I can't ride up it.
My wife has a composite, not Halfords with lockable front suspension, far better gears thumb up or thumb down, very much lighter, higher geared, and on a trip together she leaves me in the dust, calliper brakes which for her seem ample.
We have swapped bikes and no question, it is the bike not the person riding it, for road work really no point is a mountain bike.
We hear the pros and cons for disc brakes too, I will admit they do stop even when wet, but all the strain of stopping goes through the spokes so the hub needs to be larger and spokes thicker to take the disc brake.
For exercise does not really matter if you get off and push, your still getting exercise, so for me the heavy Halfords bike does the job, I tend to carry camera gear in ruck sack so not travelling light to start with, but I could not afford the price of my wife's bike given to her by my son, it was a ride to work deal, seems they get them tax free, with other intensives from work, and think it cost around 6 times price of mine. For her pushing a button to go up or down gears rather than my simple twist grip makes a huge difference. And I will admit my chain comes off far more than hers.
We are both heavy, I am around 22 stone, and it is really to get exercise so to my mind better to see countryside while exercising than look at the walls of a Jim. And those bikes in the Jim must be the worse ever designed, don't know how they can be called bikes when they don't have two wheels? Mono cycles with stabilisers which even a kid would not ride!