I am glad to see Volcarona gone. You were forced to run Blissey or Goodra on nearly every team if you wanted to prevent it from setting up, while you had to run Fletchinder if you were to have any chance at stopping it once it set up. And once you got even a single QD under your belt, you could deal with nearly all the top threats, namely Hydreigon, Heracross, and hell you could even run Psychic to get rid of Ape. While it indeed had some counters, such as those special tanks, and Mega Aerodactyl, you were pretty much forced to kill it with Fletchinder or Stone Edge. Sashed variants, such as mine was, could take any stone edge and deal some damage with Psychic, and even 2HKO if a Stone Edge missed, as it can and does. Revenge killing it isn't much of an option, as the only common scarfers (i.e. Darmanitan, Heracross, and Hydreigon) get outsped at +1, and my sashed set worked wonders against enduring a FB from Darm. I actually started seeing Scarfed Crobat (not even joking) specifically to deal with it. Being at 130 speed already, that speed is otherwise complete overkill, and Crobat sorely misses its Band or Sludge when it must counter Volcarona. When a Pokémon causes such restrictive teambuilding, annihilates revenge killing minus Fletchinder, and can blow shit up that isn't Blissey or Goodra, it is more than likely broken in the tier.
As for Manaphy, I can see it being broken as well. Though I have not used it yet, it sits at a very good Speed tier, has 100/100/100 bulk that is good for UU mons, and has access to one of the best boosting moves in the game. Similar to how Volca forced you to run a Special tank/Fletchinder among other things, I can see you being required to run Scarfmons to deal with Manaphy. It can tank any Grass- or Electric-type attack there is to throw at it, such as:
252 Atk Iron Fist Infernape Thunder Punch vs. 0 HP / 0 Def Manaphy: 170-200 (49.8 - 58.6%) -- 74.2% chance to 2HKO after Leftovers recovery
Then we OHKO back with Surf.
Due to the lack of good Electric- and Grass- mons in the tier, save for possibly Raikou, Manaphy will be roaming free. Though Raikou does OHKO and outspeed, do you really want to be forced to run it purely for the sake of killing Manaphy? Or do you want to have to run Goodra in order to tank it? Don't get me wrong- Blissey is the catchall to all Special mons, but if said special attacker has few counters, or some counters that are not very good (Raikou and Goodra are underwhelming at times, as was determined by the viability team), then it has an unhealthy presence in the metagame. Though Manaphy can be more easily revenge killed than Volcarona, it still possesses the ability to "blow shit up", which in and of itself is not healthy for UU.