Volcarona
The thing about Volc is that it demands hazard removal as well as enough offensive pressure coming from its teammates to stop the opponent from laying hazards in the first place. Your options for hazard removal that fit offense teams are to Lati@s, Excadrill and Starmie. You may also need Magic Bounce support from M-Diancie (M-Sableye is too much of a momentum killer). And as Volc provides so little in the way of resistances and countering, it puts even MORE pressure on its team mates to take up the slack.
Depending on Volc's coverage, it can either 6-0 a team or do nothing at all. Running HP Rock will leave you walled by Heatran, running Giga Drain will leave you walled by Charizard, etc. When Volc gets countered, it gets countered HARD. It just doesn't wear down its counters like Scald or Knock Off mons can. I do like Volc's ability to pick and choose its counters, but you need extra team support to lure these counters (like Latios w/ EQ or Surf for Tran).
Overall it's a pretty inconsistent Pokemon. Volcarona isn't like Lando or Clefable who will never be dead weight. Sometimes it will thrive, and other times it just won't be able to do anything at all. It really saps your team building options, too. I don't know about being hard to use, as you more or less just have to choose the right time to click Quiver Dance and then watch things die. It's just not a reliable Pokemon who the metagame doesn't particularly favor right now.