The thing about Mystcraft is that a lot of it depends on randomness. I mean getting pages is largely a random affair, trading and searching until you randomly come across whatever you're looking for (it's not like Iron where you dig down and know you will find it, it's a lot more random than that). And then writing a stable book involves luck and randomness as well, especially if you don't have every single page you need to make an age 100% stable.
All that randomness makes it very hard to balance. Someone can find the "OP" pages in the first village he finds, or it can take him weeks to get a few crummy pages.
Besides, I wouldn't be surprised if Greg just goes "Mystcraft doesn't fit with my mod, if you want the GT experience don't use it", which I would understand. I mean if you want a hard game with GT (which I assume you want if you use GT...) then why would you use a mod like Mystcraft to make things easier? It goes against the whole idea of a "hardcore" experience.