My thoughts on Shang after recently deciding to pick him up as my main:
Is extremely competetive and can be used at the highest level by a player who is willing to put in alot of time with him but is a big, big challenge. Learning the 221,skull,skull,ex skull, soul steal bnb is just the tip of the iceberg. You have to make alot of reads on your opponent during the match and cant go into auto pilot/cant keep jump punching in trying to force the bnb. His zoning, as good as it can be, is also the most difficult to pull off successfully imo. Hes slow, his uppercut is slow as shit, and the only usable wakeup attack Ive been able to use is ex soul steal (ground skull has not been affective on wakeup for me at least) and is usually necessary to do on most wakeups. You dont have time to just look at where your opponent is on the screen and then act accordingly. You have to know/predict where he is and is going to be instanly and have already inputted the correct move for the situation. Hesitating to evaluate will end in gettting rushed down and mixed up. Basically you have to commit to your read instantly and ground skull or up skull accordingly without thinking about the input. All of his skull moves need to be muscle memory and learned to the point where the right skull just comes out of your hand at the right time with no hesitation.
I love using Shang and it feels awesome to 41% someone then 45% them with their own character right afterwards but the amount of work and losses on the way to greatness with him I expect will be many with anyone who picks him up. That being said I dont think he is a good alternate. I feel like you have to know his moves so good and that the inputs have to come so naturally, quick, and on reaction that Shang Tsung players should be just that...Shang Tsung players.
Since going full on with him I have lost a LOT of matches that I know I would have killed my opponent if I was using Ermac, Raiden, Sub Zero, Jade, or Quan Chi and have been tempted to say "Fuck this guy man, Im gonna show him whats up next match and flawless his ass with one of the characters I know I can with." But I am making myself stick with him, learn from the losses, work harder, try different skull mixups, etc. In the long run I think Shang is the most rewarding character in the game once you are able to break through and start crushing with him. But I also think he takes more than double the work and time that any other character takes.