its true that the z moves usually exhaust the pkmn after the moves but the megas have taken on a more sinister/pain-inducing sideeffect on the pokemon.
a mega that goes out of control to to insufficient trust will potentially do lots of damage ie the anime. they never did say it caused a world of hurt from them when they were controlled/trusted properly. M-scizor's pincers get heavier as time passes or the excess energy can melt its actual body. gyrados just wants to burn everything to the ground, gengar sees everything a prey/food item, glalie's mouth simply breaks and a constant blizzard is released and houdoom's feet melt from his high body temp. there are more cases similar but GF is trying its best to make it sound evil.
the descriptions given in SM are night and day in comparison to USUM
. though it seems not all are doom and gloom.