@ShadyHeart
It isn't arbitrary at all. Its a good move and you shouldn't be allowed to use it if you can't use it. Don't mean for that to sound dickish. But if everybody could do all the moves with easy buttons there would be nothing fun and exciting to learn. If Jax could get out all of his slams with ease then nobody would get hype when it happens.
Honestly though Stanky Legs shouldn't be that hard. Don't try and do it as fast as you can. Start slow. Obviously it wont come out, but then do it a little quicker. Not a lot quicker. Still wont come out for sure. But as you slowly start increasing the speed your fingers will learn it better and when you start nearing the speed at which you need to do it, it will start coming out. Inconsistently at first and then you will slowly start doing it faster and it will come out all the time every time. I feel people who have trouble learning these commands are simply practicing them wrong and not practicing them enough. If you try do it as fast as you can right from the start, your fingers wont be able to learn the command properly and you will learn bad habits. You will also probably end up doing that little spasm thing because you will tense up your forearms. Guitarist looking for speed will know that devil.
I still can't get Jax's slams out. Not even inconsistently. Just two and that's it. I will pick him up once or twice a week and practice for maybe 5 minutes. If I practiced longer I would get it relatively soon-ish. But I don't because I have a lot of other things to practice. But I don't mind that I can't do it. I don't get upset that its to hard. I enjoy the struggle. And when I have finally got it down I will use Jax purely to do that move with the intention of setting the audience off. And then never bother playing him again.
I take it you never got into Smash Melee. It has two attack buttons. Simple. Until you want to learn things like Shffl-ing. I would suggest you find that game and practice that. Its the hardest easy thing in any fighting game I reckon. It will make all combos and buttons much easier when you have learned how to do it. And it will carry over to any fighting game you pick up on in the future. Good foundations make it easy to learn anything. Well, easier.