a) they go wrong which happens very often with uprights from experience. The hose goes as does the roller belts and the motor. Also on multiple models, the clips on the can are so bad that the moment you detach it from the unit, it pours its suckings all over the floor again. All parts are expensive.
b) they suck the wallpaper off or the floor tiles up causing damage.
c) you do your back in hoovering with the damn things as they weigh so much.
Bought a Numatic Henry and have been happy ever since, and considerably better off.