I think (no hard evidence), Ursacks fail because people don’t use them correctly. And/or, they don’t get up out of their sleeping bag to throw rocks at the bear that is working on the Usack… (part of the directions for both an Ursack and canisters)
As for hanging food in stuff sacks… I did that for many years before there was such a thing as a bear canister or Ursack and never lost a single piece of food… even in Yosemite. BUT… most people have no idea or are not capable of a good hang…. AND… there are not always trees to hang from… or good trees to make a good hang…
I think most parks with aggressive bears have gone to hard sided bear canister because they are the most idiot proof way to store your food. It takes much more paying attention to detail to use an Ursack correctly… and even more to hang from a tree correctly such that an aggessive bear does not get your food.
Still, I think Ursacks have their place… especially in areas where bears are not aggressive… and for people who pay attention to the details of how to use them correctly. Ursacks do not need to be hung from a high tree branch… and… not everyone is capable of throwing a rope over a high branch… not everyone knows how to pick a good branch… and sometimes there is no tree with any branch…
But any of those methods… canister, Ursack, stuff sack hung high… are better than sleeping with your food in your tent… THAT is formula for disaster! For you and the bear…