Volcanion is pretty overpowered if you ask me, it's a pefect multirole. It can stall you, it can break through stuff, it can screw around with insane powered Scalds. SR weakness, low speed and kinda mediocre HP doesn't cover that, it gets a lot of chances to switch into. It can take a hit, kill something and switch back into a predicted water move, let's remember that Water type moves are pretty common overall. It can take other Volcanions with Earth Power too. It also has a pefect type combination, offensive and defensive wise. It's going to make Fire, Grass, Water cores a lot more relevant than before.
Another thing that works really well with Volcanion are sand teams, I didn't saw this mentioned so I'll say it. I been running a sand team + Volcanion and, it can do a lot of damage if used properly. It's easier to keep sand and Exca alive with it. You can do pretty much anything with this Pokemon, it's just crazy.
I just hope it gets suspected soon, it would be stupid if it doesn't. We just got rid of a overpowered glass cannon, and now we have a overpowered bulky cannon here... It's not logic at all.
Volcanion is very easily revenge killed and is weak to SR. It's similar to Kyurem-B in that respect, but there's more stuff it can switch into. Anyway, this means that it's easy for offensive teams to deal with. Most Pokemon on those types of teams can outrun it, even defensive Landorus-T (but are people running Speed?). It's relatively easy to pile on stuff that can KO or cripple it.
As for defensive teams, they can resort to counters like Chansey, Suicune, and Mega Latias. Its range of checks is small, but that's the case with plenty of other breakers.
Bulky offense probably has the biggest issues with Volcanion. Pokemon like Rotom-W and Latios are easily worn down. Maybe it'll get suspect tested because of this, but Volcanion is on a timer just as much as its checks are (SR).