Its failings are most obvious in situations where, say, a dominator roots all the mobs in place before you can bundle them. On an Invulnerability tanker mobs that you passed by would stay interested in you for a bit when they became unstuck. On a Willpower tanker, not going to happen.
Willpower is God's gift to scrappers and sentinels; On brutes, depends on what you want to do with your brute; on stalkers I don't have enough experience to say. When it was introduced: it was one of the few tanker primaries with a sustain power that wasn't on a very slow click. This was so big a novelty that I rolled several willpower tankers. Their shortcomings soon became fairly obvious. Holding aggro on a Willpower tanker was more work, and expectations had to be lowered. The advice in the old guide I wrote still holds good. But I came to the opinion that powers that just work without being fussed at are better. And there are several tanker primaries with sustain powers as good or better (Bio and Rad are the ones I've tried.) Those sets are also good to excellent for holding aggro. Willpower no longer holds its advantages exclusively, and as such I'd recommend something else that's just as fun, almost as tough, and tanks better.