I used that in Gen 4, but with the atk moved to hp to help it take a hit. Issues are: Scarf mons, Tesshido, Puruiru. Not to mention Fake Out being totally kick-ass this gen. Bellywag just isn't a very reliable Pokemon in general. Will it rape face if it gets set up? Hell yeah. But, realistically, how often are you going to be taking down more than one Pokemon? Not often. You really need to structure your team around Poliwag to make it work, and even then you have to ask yourself, "is it worth it?"
Scarf Poliwag lead was boss last gen because it was such a "what the fuck is happening" Pokemon. It was ballsy and objectively shitty, but god damn if it didn't work. I haven't tried ScarfWag this gen yet, but I wouldn't get my hopes up. IF a Drizzle user existed in LC, Bellywag would be a very serious threat, but there is no permarain, and as a result Bellywag is mediocre.
If you want a really underrated Pokemon, give Diglett a whirl, I promise you won't be disappointed. STAB EQ hits likes a god damn truck and 20 Speed is nothing to laugh at. I run:
@ Life Orb
Trait: Arena Trap
EVs: 236 Atk / 236 Spd
Naive Nature (+Spd, -SDef)
- Earthquake
- Shadow Claw
- Rock Slide
- Hidden Power [Ice]
EQ OHKOes a shit ton of things, Shadow Claw OHKOes non-Physically defensive Missy (and 2HKOes it even if Missy invests!!) and Gastly, Rock Slide covers Fliers like the (nonexistent atm) Mantyke and HP Ice 2HKOes Evo Stone Gligar. It has its flaws, obviously, but the free kill against Tentacool and revenging Slowpoke or Wynaut with ease is just awesome. If you don't think Diggy is underrated, then just think about how few you've seen on ladder. Try him out, seriously. PLUS, he revenges Bellywag!!