Guys, as a FG keyboard players for 14 years, I have to say it is easier than you think.
Most old games have PC version. Virtua Fighter 2, SF2. And then came the WinKawaks Emulator that you can play all those arcade games. So WASD for Fighting game is very common for PC users 
With the keyboards, the most important things is to get a good one, because crap one will allow only like 2-3 buttons pressed at a time. If you can grab a mech keyboard, it has no delay, hell, i can even do akuma infinite with it without plinking (yes you can plink with keyboard too. Double tap is not that easy though lol).
With SF4, you can do almost every moves. The easiest ofc are the QCF/QCB one. Even the HCB/HCF can be done with ease due to SF4’s easy input mechanic (Just go DB to F, or DF to B, without true D input and still fine). But when you have to do something like Adon Jaguar kick, then it become horrible (require true D input). Basically you have to hold B or F then do QCF/QCB. But since it is SF4, you can go QCB + QCF and the games still recognise it as a HCB/HCF. Try GG x2 Reload or KOF/CvS2 , you will get mad with the keyboard due to the “true D” input required.
And of course not to mention 720. With 360 it is easy, you can do it the chickenwing input way. But 720 is just no and no, it require a different skill which I cannot do, but for some effing reason my friend can do a 1080 (Tager Overdrive in BlazBlue or something) with Keyboard… The true 360 input in Hugo Meat Squasher is troublesome sometimes, but you can still pull it of quite easily.
Tiger knee is harsh too, some can be pulled off with less efforts like Cammy TKCS or Sim/Gen Air U2, Oni Air U1. But instant air move like Makoto tsurugi, Adon jaguar tooth, viper burnkick will require you to go “hitbox layout” and a very, very damn fast hand to pull off.
But for the DP, everything is so, so easy. With good keyboard, you basically hold DB and then mash F + P. End of story. Want to move and buffer DP? Hold Forward and Tap down for teabagging and press punch whenever you want.
The last things that is really hard to do is Guile/Vega U1. I have to use 2 finger to hold D and B, another 2 to tap forward and then Up ( Up + forward if you want Vega U1 to hit on the way up). Since you are holding DB, pressing F will result in DF, let go of F will return to DB, tap U will result in UB. Quite easy put require…lots of fingers.
Finally, plinking with Keyboard is just like sticks, except you need good keyboards. Crap one will not recognise the plink if you dont release the previous button (plinking HP~MP but have to release HP split second after pressing MP) since with some you cannot have 2 button recognise at the same time.