Well, the thing with Scorpion is he's pretty fast, he has a screen wide incapacitate that also has the benefit of bringing the opponent in for melee, he has a teleport attack, an easy to execute unblockable, an air grab, and a moderately high damaging XRay that can hit the length of the screen. A lot of his attacks have drawbacks that can be exploited by even slightly experienced players and his full out juggles aren't extremely simple but between newbies I think he's the most overpowered.
Sub-Zero on the other hand is slow moving, has no teleport, no air grab, his incapacitate has a long startup and simply freezes the target in place, his ice clone is hard for new players to use effectively (they usually just drop them for cover every chance they get and spam frost bolts), and is unblockable has an extraordinarily short range and even a longer startup than his frost bolt. His Ice Slide is pretty descent but has a very long and punishable cool-down animation.
I'm not saying he's worse but Sub-Zero is definitely harder to use then Scorpion. Once you get used to his speed, how to use his traps, and learn how to freeze cancel I'd say he's higher tier than Scorpion but still... harder to use.