From what I can gather, I guess it just depends. Like the use of pepper spray against criminal assailants, sometimes, for some unknown reason, it just doesn't work. Just makes 'em mad.
For example, one of our neighbors has a 6-month old Border Collie pup that loves to chase cars. The neighbor asked me if I knew how to get him to stop.
"It's fairly easy," I said. "Go down to Sportsman's Warehouse and buy the heavy duty bear spray. It's gonna cost about forty-five bucks, but it's worth it. Be sure you get the real heavy duty stuff...not the little canister that your wife keeps on her key chain. Then, get in the car and drive by your place so the dog will give chase. Then have one of the passengers armed with the bear spray and spray the heck out of the dog, making sure you get him in the mucous membranes...eyes, nose. You'll probably have to do it about two or three different times, but it works."
Well, they tried it. Sure enough, ol' Hank gave chase and they nailed him good right in the face. He stopped, shook his head a few times, tried to rub it off...but then he was okay and ready to give chase again.
Like I said, for some unknown reason, it seems to effect some differently than others.
As the poor guy said, I guess it's better than nothing. Still, whenever I'm in bear country, I take a handgun in .45 Colt and a stubby rifle in the same caliber, both loaded with fairly substantial loads (300 grn. J.F.N. 1325 F.P.S.).
Even with that, though, I still keep my fingers crossed and pray that I don't have an encounter.