aaHP, cancel, aaHP works on male and female ninjas (with female, you need to start the first aaHP "closer" to your opponent), Jax, Unmasked Sub-Zero, and Stryker. All other characters (except for Shang Tsung) require you to do the aaHP, aaHP, whiff aaHPs, riot gun without a cancel.
Liu Kang requires you to take a step or two forward before starting the riot gun after the popup otherwise it won't catch him. Same applies for Sheeva and Shang Tsung.
That's what I can tell you off the top of my head.
If there's a bunch of delay I go with whiffs over canceling on any chars. If you get good enough at the timing with whiffs then you shouldn't have to worry too much about it except riot gun distance on the smaller boxes.
anyway all i can seem to find on semi vids are against jax or sub.. which are pretty easy to do . does anyone have any vids of the stryker semi infinite on chars like the female ninjas, kung lau, robots. Also i've started running in after the 1st riot gun it seems to help is that wrong.