The problem comes when you don't have sand. Volcarona is a complete monster, and if you're running a no weather team and you come across sun, then it's basically good game. Sand is the most common weather, but Volcarona can still do some major damage to sand, and if it gets 2 Quiver Dances (LO Offensive set) or 3 Quiver Dances (Rest set), it can outspeed Excadrill and basically sweep sand. Getting 2 Quiver Dances really isn't that hard, bring it in on something like a Ferrothorn and force your opponent to predict you. If they predict wrong, you will most likely be forced to lose at least one pokemon and/or get swept. The really annoying thing about Volcarona is, it beats almost every special wall not named Blissey / Chansey (which it beats in sun).
Someone has even used a Volcarona in rain against me. It learns Hurricane, so it can still destroy some pokemon, and hit common "counters" such as Dragonite very hard.