Honestly, some mega Pokemon, such as Metagross, are AWESOME - if anything, he is a defense line for all the stupid overpowered fairies and dragons GameFreak released in Gen 6: Gardevoir, Altaria, Diancie, Lati@s and even Zard (110 > 100 speed) and he was also able to stop Tapu spam in Gen 7 (I'm more content with that than megas).
Then GF decides to remove the speed limit with mega pokemon and MMeta is gone. He was easily the best designed mega evo.
The rest of them are defined by Mega Ray, Mega Mence, Mega Zard X & Y, all fairy-type mega evos, all these ultra-fast Gangnam Style mega-evos (Lopunny, Beedrill and Alakazam especially - but Sceptile I like, maybe Aero too), Mewtwo, Venusaur, Blaziken (lol), Garchomp which are just plain jokes.
The ban of Aegislash also contributed significantly to the mega evolution crapfest.