In the test report of 2004 according to Ursack:
Bears carried inadequately-secured Ursacks short distances suggesting that users should be able to locate most bags that might get carried off by bears. Distances carried during the four tests were 0.3m, 1.6 m, 5.8 m, and somewhere between 41 and 67 m.
In those cases, the sack did prevent the bear from getting to the food in the sack.
Sierra Interagency Black Bear Group (SIBBG) did investigation on Ursack failures during the 2007 season at Yosemite.
SIBBG did not conclude that the six Ursack failures in 2007 were caused by the same bear.
Over the past 10 years Ursack have been improved. However, just as bear canisters fails, Ursack is also prone to occasional failure.