One button Hadoken would be utterly WRONG. I'd almost be willing to bet that nearly everyone that's having an issue popping it off reliably is using an XBox controller with the circular, 9-'block' d-pad. I'm thinking the diagonal doesn't read as diagonal all of the time. I haven't had the time to actually TEST that theory, but I have a sh'ton of random controllers I could try. x.x
The Logitech F310 that I use is with the XBox layout. I'm not sure how much the diagonal is accounted for into the programming, but my controller handles it okay either way. In X and X2 the diag works just fine, so it's really about practice and timing. It was not great in the early versions, but it's been pretty consistent in all of my playthroughs since it's been re-tweaked.