I mean sure if the base dice was d20 that could mechanically justify the HP drain since it would be much stronger than similar abilities.
What I really don't understand is why barbs are being singled out here. Other imbues could have thematic restrictions but they don't.
EK doesn't drain SP per hit (or get burned from all that fire on their sword), KotC doesn't only work on undead or outsiders, rangers imbue doesn't only work on their favored enemies, etc.
Not suggesting they do that but if they don't why the self harm on barbs?