"To enter one, you leave the Astral Plane, forming a new physical body (and equipment) on the plane of existence you have chosen to enter."

That is what makes Astral Projection broken. Buy one +5 Tome for each stat, give them to one party member, cast Astral Projection, have everyone in the part get +5 inherent bonuses to every stat.

It's unlimited, free, consumable items. Without that it's inline with the other 9th level spells (powerful but not broken).