Well, the combination of all this stuff plus some other stuff makes it hard:
The pro moves, they are done inputting a specific button combination during a special move, and getting the timing for that is hard
Putting a three-hits combo together with a special move afterward, something like square square triangle back back triangle... Sometimes that is two special moves after the combination, or sometimes a pro move, or sometimes two pro moves...
Keeping the opponent on air with single button attacks, somethings like square, X and triangle, which will not connect at all so you must time the attacks precisely or else they won't happen
The opponent must not touch the ground at any moment of the kombo (some special moves like kahn's hammer don't count on this though)
Sometimes the move is impossible to pull unless you're close to a wall, and some are impossible to pull if you are on a wall, and some require you to be far away from the wall then get close from it on the final of the kombo
Let me give you some examples... Shang Tsung's 9th kombo: Back down circle + up circle, Back square, Back down circle + up circle, Back square, Back down circle
With back + down + circle, brother Shang will pull a slide and kick the opponent in the air, if you PERFECTLY input up circle in the PERFECT moment, brother Shang will throw him in the air even higher, after throwing him very high in the air, you must do the back square punch while he is falling, in the right moment of his fall or he will bounce to far or will go straight down, and after you punch him, you must get to the ground, do back + down + circle again and slide beneath him AND DON'T LET HIM TOUCH THE GROUND, if you manage to pull this off, good, you still have to perfectly time the pro move once again and throw him in the air once again, after that, you have to jump again and punch him with another back square punch, but this time he WILL bounce too far, no matter what, so you have to be close enough to a wall, if you are too close, the next slide won't connect and if you are too far you won't hit him, he will touch the ground and it's over, so you have to make sure he is in place since the beginning of the kombo, and if you manage to pull the last slide before he touches the ground, congratulations, you just got the third hardest kombo in my opinion
Okay, that was enough, i'm not even touching Raiden's 10
Flow -"The important is to pwn other ppl"