[Siven]

Granted Foe Lullaby is a lv27 song, so you can't really do this for something like Giddeus, but would BST/BRD work better than BRD/??? for higher level BCNMs? You'll lose out on not having Soul Voice, so sleep duration'll be shorter. You can't use Mary's Horn, but your built-in CHR will be slightly higher, not to mention more HP and actually decent melee ability. You can also take out at least 2 accompanying mobs with charm. I've just not seen anyone talk about this particular combination, so there may yet be some hidden catch.

[StarRealms]

BRDs having a singing skill like spell skills, also song effectiveness is usually halved without an instrument. Although I have never personally I tried it, I imagine that a BST/BRD would not even be able to put the enemies to sleep, because of the much lower singing skill. (For comparison try subbing a whm with a lvl 40+ melee and using weakening magic on an even or above enemy-- it's resisted most of the time)