Thank you for bringing this up, idkup. After extensively using both Lati twins last season, I can safely say that they are both worthy of an A placement. I did encounter several KeldTar teams, but even those did not particularly hinder the performance of either one of the Lati twins. Weavile teams have to play risky games and either sacrifice a Pokémon to bring in their Pursuit trapper or hope that Latios or Latias is clicking Defog or Recover/Roost.
Latias' utility and bulk is always appreciated and, unlike Latios, it does not always have to heal up after switching into Mega Charizard Y, Keldeo, Volcanion, etc. Healing Wish Latias is particularly good on dedicated offensive teams that appreciate having a check to specific dangerous threats (such as the ones I just mentioned) that also happens to possess Defog and the ability to give one of its teammates a second chance.
As for Latios, I have been an avid user of the Life Orb variant, as I do appreciate its ever so slight increase in power (in comparison to Soul Dew variants). It still manages to chunk Heatran and Ferrothorn quite nicely while obliterating the likes of Zapdos, which might attempt to Roost up on its powerful Draco Meteor, if they happen to be faintly weakened. Choice Specs sets are quite prediction-reliant but can wallbreak rather effectively and are a threat to all teams. I am less sold on Choice Scarf Latios, because I would then prefer using Latias, but I can acknowledge the occasional worth of such a set.
I do not particularly find Mega Alakazam to be overrated. I only faced it a couple of times, but it managed to land all its hits while sometimes lowering the Special Defense of my main checks for it. I think it is still dangerous in a metagame where many teams (mostly balanced ones) are relying on their defensive Mega Alakazam checks without necessarily having any priority move in the back to handle it if it ever gets out of hand (e.g., Calm Mind, Substitute...). From personal experience, Mega Scizor was also surprisingly uncommon last season, at least on Silver server. Oftentimes, players find themselves in a position to have to hope that Mega Alakazam misses Focus Blast against something that could KO it back.
As for Mega Medicham, I was not impressed by its performance. I think its frailty and relatively lackluster Speed are definitely holding it back. Slowbro has been increasing in usage and I have even encountered some Mew sets that can always survive its hits. Overall, I do think it might be the weakest Mega in A+, but I am open to other opinions.
Finally, we get to Mega Diancie... I still find that it struggles far too much against all archetypes to be considered on par with other A+ threats. Chansey is still one of the most used Pokémon in the metagame and Ferrothorn gives it too much trouble on balanced builds. I will admit, however, that it makes it easier to anticipate certain switch-ins to try to take advantage of them. Protect is also a double-edged weapon, as it allows the player to scout for Choice-locked users while leaving the outplay door open for both parties.