I think the reason why we talk about G/F/W cores in UU is that in OU they are totally outclassed by a better core; steel/dragon, one with levitate. As such tons of Grass, Water and especially fires (due to SR weakness) have been moved down and so G/F/W in UU has alot of options.
In OU grass has celebi, breloom and roserade of which celebi is the only truly bulky one. Fire has heatran and infernape; infernape especially isnt very bulky (heatran can be). Water has plenty of options.
In UU grass has leafeon, ludicolo, sceptile (not bulky enough?), venusaur, tangrowth, torterra and other NUs. Fire has Arcanine, Blaziken (probably not bulky enough), Houndoom, moltres and other NUs such as Entei. Water has even more options than in OU.
So i am saying that it is strong in UU because it has versatility, that in turn is because it is not great in OU, so the G/F/W pokes are being moved down.
Looking at the Jan server stats; the top 15 pokes has only one grass, one fire and one water poke. There are however (amongst others);
2 psychics
3 ghosts
2 grounds
2 normals
Ive said before that dark/ghost combinations are the way to go, and i still believe it; dark has a good match up against psychics and ghosts and a levitating ghost does well against grounds and normals. Unfortunatly the quality of darks/ghosts in UU are questionable.