Actually, Slowbro's isn't that bad, not to mention that it's pretty fitting to the design. Shell Armor will make Mega Slowbro an even greater Calm Mind sweeper, plus, Slowbro already has reliable recovery in form of Slack Off, Regenerator is kind of useless if you're attempting to boost up your stats and sweep, and anyhow both Slowbro and Mega-Slowbro will still be pretty viable as physical walls, regardless.
Sharpedo's ability isn't bad either, much better than Speed Boost, actually. Why? Because you can get 1-2 boosts before mega evolving by revenge killing and protecting, and at that point, you won't need any more speed since it already has a somewhat high speed stat, which, with the boosts, will allow it to out speed most mons in the metagame. Plus, Strong Jaw will make Sharpedo's Crunch EVEN stronger, not to mention that Ice Fang will become somewhat viable with the boost.
The only Mega with a bad ability so far is, imo, M-Swampert (and M-Sableye, not that Magic Bounce is a bad ability but Prankster was superior), because running weather teams isn't viable on higher tiers anymore and it's overpowered, and therefore banned, on lower tiers. So unless Primal Groudon/Kyogre's new abilities happen to summon permanent weather, Swift Swim will be completely useless on Swampert, even more useless than Gallade's Inner Focus.