What I think would be a problem is just simply how easily the BD variation is setup in any situation really. Its defenses/typing also allow it so its not as vulnerable to priority moves as we would believe actually resisting at least 3 of them. You wouldn't even have to run sub if you're daring enough since most moves come short of 50% on it you could chance a Salac.
As for in general it has everything you mentioned including an option of actually being mixed. (It can get at least 250 on both attacks which is quite good and viable when you have a 150 and 120 STAB respectively).
When did 250 attack stats become good if you aren't weilding Life Orb? Nevermind in order to achieve it Poliwrath needs to pump EVs into an SA that isn't worth much. Hydro Pump and Focus Blast are innaccurate and won't even take out the Electrics or Grassers that come in, and Focus Punch only has 150 BP guarenteed if you're behind a Sub. It is not at all difficult to do 50% to a Poliwrath that hasn't invested a single EV in defense. Waterfall and Brick Break are grossly underpowered, and without a Salac boost Poliwrath is easily outsped.
To illustrate:
Lets say Poliwrath just switched into a CB Waterfall from Seaking(FUCK YEA!)
Poliwrath used Substitute
Opponent switched to Meganium!
Poliwrath used Belly Drum!
(Attack Max, Salac Activated)
Meganium used Giga Drain!
Poliwrath's Substitute faded.
Now: here are all possible options against 212/148/148 Calm Meganium (354 HP/273 Def/300 SDef):
Waterfall: 48-57% damage
Brick Break: 90-106% damage (OHKOs half the time, Poliwrath gets finished off half the time.)
Ice Punch: 121-142%
Rock Slide: 60-71%
Earthquake: 40-47%
Poliwrath gets an easy out with Ice Punch, but Ice Punch + Waterfall leaves it susceptible to Bulky Waters, and Brick Break + Ice Punch isn't as powerful.
Lets go with 252 HP/84 Def Impish Quagsire:
Waterfall: Immune
Brick Break: 88-104%
Ice Punch: 59-69%
Rock Slide: 29-34%
Earthquake: 78-92%
The recurring pattern here is basically that Brick Break is the only thing Poliwrath can OHKO with, and its a shaky OHKO at that. Now, my calculations have been using 252 Atk Jolly so that Poliwrath can get up to 262 Speed.
Check this: 252/252 Impish Gastrodon:
Waterfall: 85-99%
Brick Break: 79-93%
Ice Punch: 52-62%
Earthquake: 70-82%
Not even Waterfall OHKOs it. Better hope for a flinch.
Now, for Revenge Killers you have Scarf Manectric/Chan/Lee, Shadow Sneak Banette (who incidentely, can come in on Brick Break.) Basically anything with Focus Sash, Choice Banded Sucker Punch (Mightyena+), Extremepseed Linoone, Endversal Ape/Zangoose/Lee, Mach Punch Hitmontop...
In short, there are tens more revenge killers for BellyWrath than there were in Advance, and since Bellywrath is generally a one-shot setup even if you can Wish it back to health, its prospects aren't great.
I like Poliwrath as much as the next, but he doesn't cut the OU Mustard very well and doesn't unbalance UU in any way. We're already saying "BL is to big," so I fail to see why we should take a perfectly viable defensive UU pokemon and bump it up because there's a small chance it might be able to Drum up and kill a single pokemon.