Compared to every other Guitar Hero game and Rock Band, GH3's hammer-ons were veeeeery forgiving. They gave you a very wide margin of error in your timing. Guitar Hero 2 and Rock Band are more strict, requiring you to be a little more precise. Guitar Hero 1 hammer ons were almost impossible.
Personally, I kind of enjoyed the forgiving nature of the GH3 system, just for the way it made seemingly impossible songs pass-able. Through the Fire and the Flames would be three times harder on the other games.