In USUM, several pokedex entries suggest that mega evolutions are terribly painful to pokemon and make them uncontrollable. e.g. Gengar becomes a serial killer when it Mega Evolves, trying to kill anything or anyone, even its own trainer.

I was surprised reading this since mega evolution seemed comfy in the games I played so far. Are these dex entries even canon? Has the negative side of mega evo ever been explored as a plot point?