I know how much Mega Gyarados suffered there, it's Mega gains Dark-type yet it has no usable Dark moves to take advantage of. The only I recall is Bite, and that's weak in base power meaning it can't make use of the Dark STAB.
Ampharos lacked special-based Dragon moves until now, when it gained Dragon Pulse to take advantage of its Mega's Dragon typing and boosted Sp. Attack. Pinsir, despite becoming part-Flying in Mega form may not learn a good Flying move, but its Mega does get a powerful ability that makes its Normal moves Flying-type, effectively giving stuff like Return and Quick Attack STAB.
I really want to see the issue with Gyarados fixed, and give it a stronger Dark move. Learning Bite but not learning Crunch feels so weird, especially given most Pokemon with jaws were able to learn both moves. Sceptile does get a few Dragon moves, but it needs easier access to them because not everybody wants to breed like mad to get egg moves.