I used them for a while a couple of years ago. They certainly help with accurate Deep Strikes. Particularly for something with a large base like a Dreadknight. The problem is getting them someplace where they are useful. With the FAQ stating that abilities don't radiate from a vehicles hull, the Mystic and his unit would have to hop out. Since Deep Strike reserves occur before disembarking units (at least that's how I understand the order of operations), the mystic would have to be exposed to fire for at least one Game Turn. Not to mention have survived the journey to get to that point.
In the end, I gave up on the concept because there were too many points of failure for it to be reliable. And that was before the FAQ. It'd be even worse now.
Not saying he's useless. I could see a case for defensive deployment but he's very situational.