After having tried out MAero for a bit I and could definitely see it dropping to B+. Honestly the big problem with it is the same problem with Lando-T atm : the metagame is bulkier than it used to be, and MAero does not appreciate being in a bulky metagame. It is very good against offensive teams, both as an offensive powerhouse and a check to scary stuff like Lopunny, Talonflame, Tornadus-T and Pinsir, but Offense isn't really common at the moment compared to bulkier teams that easily handle. There's just a lot of bulky stuff that can take its hits pretty well running around atm. Belence is pretty much the dominant playstyle atm, and it's one MAero struggles a lot against.
And yes, there is the Taunt+Roost set which puts in work against bulkier teams but that set lacks so much coverage it hurts. Really, any set that runs Roost is going to lack pretty crucial coverage, and I feel like 4 Attacks is just better in this metagame, even if it does mean Aero doesn't check the stuff it's supposed to as consistently. Then there's the fact that it needs to have MEvoed to actually check Lopunny, otherwise it gets outsped and takes way too much from HJK. There's a lot of stuff MAero can't mega against safely, and it doesn't help that a good number of them are great partners to Lopunny (Jirachi, Phys Def Garchomp, Ferrothorn). That being said, it's a good answer to 2 very threatening Pokemon for balanced teams, Gliscor and Landorus if running Ice Fang (even Aqua Tail OHKOs Landorus after SR). But when you put it in the same rank as Tornadus-T, who manages to be effective against multiple playstyles, it doesn't seem that good in comparison.
Concerning MTTar, the problem with it isn't necessarily its effectiveness but how much competition it has for its role. Specifically from Mega Altaria, Charizard X and Mega Gyarados. Mega Tyranitar does have its perks over these like setting up on Latis, not being revenged by Talonflame, and other neat stuff but overall it's harder to build around, harder to fit on teams, and harder to sweep with, mainly because of its 4MSS and its main STAB having bad accuracy which, for a sweeper, is a huge problem. As a result, it's really hard to justify using as your mega at all unless you're specifically building around it. Even though in a vaccum it may not seem inferior to MSharpedo and MHoundoom, I can assure you that I have seen far more good MSharpedo teams than good MTTar teams, mainly because MSharpedo is a lot more friendly to teambuilding and, in practice, actually manages to pull off a sweep more often. As for MHoundoom, the offensive and defensive synergy it brings to teams makes it a pretty neat fit on a lot of teams and a nice Pokemon to build around. Mega TTar on the other hand, doesn't really bring that much actual support to a team, especially when compared to band/scarf/smooth rock TTar. It's just a tricky Pokemon to use not because of its stats or even its typing but because the metagame that surrounds it is really unfriendly towards it.
Also I'd like to nominate Kyurem-B for A-. Kyu-B really seems better than everything else in B+ to me. It's particularly strong against balanced teams which often employ ice-weak cores like Celebi+Gliscor. It's very versatile and can pretty much decide to break past what it wants depending on the team, which enables it to lure certain Pokemon, for example HP Fire lures Ferrothorn/Scizor for a potential Diancie sweep, and Iron Head is a legitimate option for luring Clefable which can help out a ton of stuff. It does struggle against offensive teams, but it can still run a Scarf set which can clean weakened teams thanks to Outrage. I find it to be on par with Mamoswine overall, and better than Diggersby.