To add on to this, Kitana and Mileena are especially tricky since they Square Boost and Teleport respectively, making punishing an IA fireball with EX spear difficult. Actually, characters with air-ok teleports are more difficult than those without since it nullifies the long-range aspect of Scorpion's baiting game.
Also, how well does he deal with reversals like KL's spin, SZ's clone, Kano's up-ball, etc? If he doesn't deal with that well, I think Kitana could easily be his worst matchup in terms of better moves.