If the wind is up, even the pepper spray "gel" can a) miss its target, or b) contaminate you (if not get you in your face, get on your gloves or whatever, that eventually works its way to your eyes....
And, for some reason, some dogs don't react to it. I am in a lot of backyards or homes on alarms, open doors, whatever and often encounter angry dogs. Our OC is a whole lot stronger than average dog spray (it is designed to turn an angry, violent person into a slobbering mess. We were sprayed in training and it is the worst pain I have ever experienced) and is our preferred dog repellant, but on some dogs it just doesn't work. Bad day when that happens.