he's very bad at full-mid screen
This is only true when he has no meter, which he always does. He still has armor on EX shadow kick which he can use to get in, and you still have to respect even a regular force ball sometimes. Cage is NOT "very bad" full screen. He does have trouble, but saying he's very bad implies he has next to no options, which couldn't be further from the truth.
I've made jokes that Cage is one of the only true zoners in this game; that you have to respect him from almost every point on screen. Next to Kenshi, of course.
Also, the Scorpion match-up really is that bad (it's a 7-3 at the VERY least). In a game of footsies, Scorpion loses, and against someone with such deadly footsies as Cage, it simply isn't fair. Scorpion is still garbage even when he manages to get a full screen in between him and Cage: EX teleport uses a bar, has no armor (Cage can D1 it into a reset), and only puts him right back next to Cage. Oh, and hellfire/teleport/spear mix-up is incredibly risky.
The ONLY time Scorpion is in control of that match is when Scorpion has Cage in the vortex, but so what? Scorpion has to get Cage in the vortex first, which is one of the hardest things Scorpion will ever have to do, and usually by then Cage has a breaker from Scorpion blocking all of his bullshit.
Scorpion has to make virtually perfect reads against Cage or get blown the fuck up, and that is why it's such a fucked up match-up. I'm not saying it's unwinnable, but a smart Cage can almost always bend over an equally smart Scorpion.