I do agree with that, Shang himself doesn't have much of a learning curve in comparison to someone like Kabal. However, if you want to do very well with Shang it's obviously encouraged to learn good morph combos, which requires investing time into other characters which you may not have much of an interest in. Though with the recent news of all of his strings being duckable and morph leading to unblockable combos right as he comes out of it...he clearly has some HUGE flaws.
It's funny because I was just thinking of Kabal too.
Thing is, there is no Shang morph combo that you should capitalize on learning, and that takes away a big part of his supposedly "high" learning curve.
Most of his morph combos (those who start out morphed and end with a soul steal) are really situational, more than any other thing I can think of in this game:
1. You'll never land a combo as soon as you morph. Not with EX SS or regular SS.
2. You'll never have enough time and space to land that morph combo that starts just 2 seconds after the EX SS.
3. I can't really be sure on this, but I think you're at frame disadvantage when you morph(46f after SS's first hit). So when you morph you're bound to an attack, and when you unmorph you're gonna get bodied as well.
4. Even when the morph combos weren't situational, you'd only have to learn only the characters that are bad matchups for Shang(or those who outzone him). With the easy dial-in combos and easy-input in MK, that's not a big issue, so it does not suppose a high learning curve.
5. Thing is, though, combos are really situational, so you shouldn't be trying to combo, but to use pressure strings(which are safe launchers in mk, lul), which will result in learning for your opponent to back out, and by the time you're applying your pressure, you'll unmorph and get a taste of your "own" medicine.
Soul steal does add some in-depth to the game, but it's not that killer move that a lot of people portray it to be. It has it's flaws and I can imagine it's really hard for any game studio to implement and balance well.
Other stuff that make him have a high learning curve? Well Shang is one of those characters that have a theorical use of all of his strings. Antiairs, footsies, pokes, zoning, counter zoning, etc are just as hard as with any other character in the game. Only thing that really implies thinking outside the box is his zoning. Being it so slow it's not easy to catchup against other character's zoning. Plus, zoning with his soul steal requires a lot of thinking, but the payoff isn't as good as we can see by the points explained above.
Also, Shang combo's are not hard in any way whatsoever. Wanna continue the pressure? Use upskull. Then the generic combo is either b2, f3 mixup or launcher into mid, far and ex ground skull. It only requires adjusting to the timing at first, and that's as hard as it gets.