A frontal dumbfire is like a slap in the face. Not only does is most certainly sting, it's also quite an insult.
In wcp, I am 99% accurate with dumbfires. The trick there is patience. In wc3 & 4, I'm about 45% accurate. Though, once, in wc3, Vaquero was RTB, and a Vaktoth was hot on his tail. So, I lined up on V and toggled my dumbfires and waited. Sure enough, about five seconds later, that Vaktoth was directly between me and V, so I sent a rocket his way. Crispy Kitty Critter Litter.
Then there was that time I killed a Vaktoth with decoys, but that's a different story (it takes quite a few, but it actually works)