Yes, I agree it is a bit overpowered...but then, there is the argument that it is entirely up to the DM whether the character gets his/her hands on one of these things. Any hints on what enhancements to remove to reduce the power?
If the weapon is destroyed, the character instantly loses all supernatural abilities granted by the prestige class (i.e., everything that's not a save bonus or a feat; it is like becoming an ex-paladin in that respect) and also has to make a Fortitude save at the time of the weapon's destruction (DC 15+1 per level in Bsw) or die instantly, due to the shock of having half of one's mind ripped away. If the save is successful, the character instead loses 10,000 XP (DC Will 20+1 per Bsw level for half).
Naturally, since the weapons are almost always powerful magical items (frequently artifact-level), they are appropriately difficult to destroy. See the Dungeon Master's Guide for details.
P.S.: What is BAB? What is PrC? Also, the only 3rd Ed. books I own are the Player's Handbook and the Monster Manual, so referencing me to more specialized materia probably won't help.