You're really jumping the gun on Mega Lop and Diancie. Diancie is 4x weak to BP, but it checks BirdSpam, Earth Power gives it decent coverage, and it's 700 BST is very well distributed. Even with lowered defenses, 50/110/110 isn't terrible, and it's 110 speed tier is great. Having a 4x weakness to a rarer type isn't bad, especially when said type really only has two users that can hurt it (ScarfTran and Mega Scizor). Having a 4x weakness isn't a fatal flaw by any means, by that logic MegaMence and Mega Swampert have crippling flaws as well. Ice Shard isn't holding Mence back, and that has more users in OU than BP does. Also, Lopunny and Medi really don't share any common traits outside of HJK. Medi and Gallade are closer comps (still not good comps, but still closer). Medi is a slow wallbreaker, Lopunny is a fast attacker with 2 strong stabs and a great support movepool.
Yup, it does check birdspam but right now I'm viewing the negative sides of the megas. I haven't actually used them yet so I'm making teams based around them to see what their true potential is. What they counter, their effectiveness...etc.