While failures have happened, they are rare (yes, I am a huge fan of Ursack bags)...and almost always are due to the human not closing the bag properly. If the failure did actually happen, I'd wonder - considering that a bear shouldn't be able to get to the opening if tied off to a tree!! The bear would be able to pull at the bag from the rear - with it's teeth.
I am always suspicous when a ranger is the one selling the tales. Why? Here in Wa, at the ONP, there is a ranger that tells tales to force bear canisters on hikers. His freaking tales change constantly! They are always about how Ursacks are not good enough. We blatantly lie to him at this point. Ursacks are allowed everywhere in that park except for the coastal strip, so they go with us. We obey the coastal rule. But when a story ranges from insane mt goats to drooling bears, and it changes on every visit, you have to roll your eyes.
Yet the park service there actually has Ursacks that they use themselves!! Some of us ran into him in the backcountry-and did he have a canister? Nope.
Though I can understand the point of canisters. If everyone used one, they'd have almost no incidents. It is easier to just require them, than to rely on humans to use a tool properly, which is what an Ursack is.