Astral magic seems overpowered

[Tolkien]

Astral does have access to ridiculously powerful battlefield-wide spells. However, such spells require a massive amount of slaves/boosters/gems, and a good antimagic or SC makes the whole thing useless very ineffective. Soul slay, enslave mind, and paralyze are great counters, but blood and death have even more effective counters. Claws of Inferno/Cocytcos for example, while very high level (and hellbind heart), are awesome anti-SC/thug spells. Drain life also, as it isn't resistable (unless lifeless).
But in general though, astral took a really big hit when it lost clams, which very much evened the field. Astral has some really powerful nation-only summons, like Seraphs or Devatas, but those require ridiculously high amounts of astral pearls, and pearl income is limited. Golems are also vulnerable to a good ol' magic duel. Other magic paths have much more, and much more efficient, summons. Death: Tartarians (still ridiculously good). Blood: hell, everything (with slave income in the 100s, your not really all that limited) from demon lords to heliophagi to archdevils to armies of demons.

But in any event, communions are counterable. Earthquakes are the bane of all communions.

But anyway, astral is good, but not imbalanced or anything. I personally feel blood wins purely on the basis of powerful summons and massive income.