It doesn't say power weapons can't be daemon weapons, however I noticed if you make a power fist into a daemon weapon it has chance of being even more powerful than a multi-melta.
Normal (Str 60) 2d10 + 12 E Pen 8
Bound Daemon (WP 40) 2d10 + 16 E Pen 12
Hell Spite 2d10 +28 E Pen 12
Is this unbalanced?