Then again, it could be said that if you died in TAR, you soul is in a neutral place akin to limbo or purgatory, and you might actually be further away from the soul pool. It really can go either way. I think we did see a corruption of TAR - it started out as being a place with unseen eyes looking at you, but towards the middle of the series, the characters started remarking that it seemed threatening and made them nervous. As a reflection of the real world, it seems likely to me that as the Dark Ones' touch on the real world grew, so would It's influence over T'A'R and that would feed on itself making It more powerful in both realms.

It's really up in the air. Transmigration itself is so rare that we don't know anything about the mechanics of it, let alone the limitations. All we really know is that some Forsaken were given new bodies and others could not be given them. No idea why.