Mega Slowbro's effectiveness has been questionable lately. It's been mostly outclassed by Giratina and Cresselia as a Fur Coat Pokemon, courtesy of its general lack of special bulk. It is an incredibly inconsistent counter to the -ates, as most of them go mixed in the current metagame. Its Soundproof set was fantastic, but Techno Blast became so common that it compromised its viability. I would limit its use to Prankster and Poison Heal.
I personally find this a little tough. Due to its weakness to revenge killing, Primal Kyogre is best used as a sweeper in defensive teams. It's clear that with the amount of support Primal Kyogre needs, it should be A+ rank, yet the amount of pressure it provides alone with its required amount of support is staggering. It has the ability to single-handedly defeat almost every team in the Balanced Hackmons metagame. I think it can easily stay S rank.
Aegislash has always been a fantastic pivot to some of the most dangerous threats in the Balanced Hackmons metagame, but its effectiveness as a counter to -ates has been lessened, as the two most common -ates in the current metagame break walls a lot more easily than their predecessors. However, Aegislash has also regained its use as a consistent Magic Bounce Pokemon, as Magic Bounce Pokemon are no longer reliable against -ates, which allows Aegislash to have a more dedicated role as a "utility counter" to various setup sweepers. You could say its Prankster set is also better in a sense, since Magic Bouncers, Poison Heal Pokemon, and walls in general have been uncommon lately.