Pokemon that would most likely be made useless with Mega forms.

Marowak is highly reliant on Thick Club to deal damage. MegaMarowak would need to have titantic native attack and hopefully a fixed speed stat to be worth using over a standard Thick Club.

We don't even know how exactly Mega evolution is triggered so it could be a huge gamble in it's own right. If it's something triggered by taking damage that would invalidate the possibility of a MegaShedinja.

tl;dr: >muh bros