The trade off is that WW is completely immobilized (and thus vulnerable) during the Ult. So long as your team is intelligent enough to pick a support with rapid-response CC (Lulu and Leona come to mind), he'll be knocked out of it before he can dish out his real damage. WW stopped being played as a melee carry back when because people realized that turning him into a walking ultimate was suicidal come team fights, and any suicidal strategy is ultimately sub-optimal. It's certainly overbearing against a team with no CC, but picking a proper balance of CC and damage (team comp) is part of playing this game (and the main reason I stopped playing blind pick).
As for his synergy with current jungle items, I just don't see it as superior to his old Magic DPS builds, which were never really a problem for me or the friends I play with. Devourer is probably better that old (and by old I mean S1/S2) Wriggles, but I really doubt it's actually better than Bloodrazor. Faster and cheaper certainly, but knowing whether or not it's better will require more analysis than I have performed as of yet.
Finally, just as itemization is a part of building your offense, it's part of building your defense as well. The vast majority of WW damage is magic, which leaves
/
as a strong alternative to
(yes that's a snarky pot-shot at Jillfinger's post above), and
counters suicide-WW for the same reason it counters assassins and burst-casters. The removal of
does present problems for other carries choosing to fight him, but they still have avenues to beat him. Sadly items are about all the end up mattering for true carries, as their play style inherently degenerates into right-clicking (kiting is a function of right-clicking) with a button press (WW, Trynd, alot of Marksmen) or two (Yi, Fiora, etc.).