I'd like to know this too. I usually use either Ermac or Scorpion as they're the only two fighters I've had time to learn.
Personally, I think Liu needs to get nerfed. I've actually seen him do this combo and somehow (lag?) pop me back up off the ground and restart it, which makes it even more retarded when the entire round consists of somebody doing the same single combo from start to finish. But that's another story.
When I do a combo with either of my mains, they're a pain in the ass to pull off; like any ~40% combo should be. But when somebody connects with just one piece of Liu's combo - no matter where you are, airborne or not - all you have to do is hit the same three easy-as-pie buttons over and over again and bam, instant half damage. The combo just takes no skill IMO, it's the easiest one in the game from what I can tell and is nothing compared to those of the other fighters. There's no timing involved or precise location requirements to be met, and lag is virtually irrelevant in it's success. An outsider looking in would jump to the conclusion that the only reason his opponent picked Liu Kang is because this combo is the easiest. It makes people think you took a shortcut, and gives them the impression you can't do anything else. In my case at least, if wake-up attacks are the key to beating it that explains why I haven't been able to yet, because believe it or not, I've haven't learned how to do them. But to me, the above still stands.
Come to think of it, I have never seen anyone do a different Liu Kang combo. Not that this makes someone performing it any less cheesy, but is it the only one in his moveset or what? If it is... then at least I can half understand why people use it.