I think about a third of it is knowing the motion, and the rest is just practicing the timing. So many other fighting games use this motion that it's definitely worth learning. I found the timing to be easier in SF2 Turbo and after. I think the faster action prompted me to do it in a smoother motion (less time to think and analyze it).
The other big factor are your joysticks. Tighter microswitched sticks are definitely easier than leaf-style joysticks. One interesting test is to go into the service mode of the machine and enter the IO test. It'll show you when you're hitting it and when you're not. Capcom games seem to have the easiest input test of any manufacturers. I used it to test out my control panel once built.
In CE, I still like to do the hidden earnings report: during attract mode, on player 2's controls press U,U,D,D,L,R,L,R,Light Punch, Medium Punch. If I remember right it shows earnings per coin slot and how many times each character's been selected. Why do I remember this stuff?