I've always wondered, why not expose on Vader. He seems like a perfect candidate. It makes him essentially a ptl x-wing that doesn't take stress. And can get the extra green/die defensivly when needed.
In "normal mode", Vader works as normal: 33-point TIE Advanced, legendary for its lack of offensive output. Even if he was the right price before equipping him with Expose--which he's not, really--he's now overpriced by 4 points.
In "Expose mode", Vader works like a PS9 X-wing with no pilot ability (because you spend his extra action using Expose). He's now overpriced, in comparison to Wedge, by at least 5 points.
Vader + Expose is something a lot of people come up with after considering the problem with Expose for a while, and it looks okay--but only from the perspective of "where can I put Expose where it would do some good?" If you look at it from the perspective of what Vader + Expose can actually do, though, it becomes clear (to me, anyway) that it just makes him worse.