nameless said:
I disagree. At higher levels (12+), almost every opponent will have +5 or +6 armor bonus.
Every HUMANOID opponent, you mean. I've never seen a high-level campaign where every opponent was a humanoid with class levels. There are always a large number of dragons, undead, advanced creatures with templates, and so on, usually exceeding the number of humanoids.
If every opponent you faced had a +8 Armor bonus AND the weapon had no other downsides, then it'd be balanced at a +4 cost, since a +4 Enhancement gives +4 attack and +4 damage.
(You'd actually need it to be +12 Armor with the new Power Attack rules for 2H weapons, but let's ignore that)
Most types of opponents won't have an Armor AC AT ALL, let alone a +8. Let's see... things without an Armor AC bonus... that'd be Aberrations, Animals, Beasts, Dragons, Elementals, Magical Beasts, Oozes, Plants, and Vermin. Against all those creature types it's a useless enchantment.
Against Undead and Constructs, it's not only useless, it's a drawback; the entire weapon becomes unusable.
Against the remainder (Shapechangers, Humanoids, Monstrous Humanoids, Giants, Outsiders) it's only useful if they're wearing armor and/or using spells like Mage Armor.
Against the SMALL number of enemies with higher than a +8 Armor AC, Brilliant Energy is a better choice than putting a straight Enhancement bonus on the weapon. Against the majority, it's a far worse choice, and that's before we even factor in the negatives of BE or the other benefits of an Enhancement bonus.