Hello, I am a Lv.41 hume dark knight/ 20 theif, and yes, humes make great dark knights. Damage output is very high, since humes str is only slightly below that of elvaan and galka, plus accuracy is much higher than the elvaan dark knights.
I've partied with elvaan drks on a few occasions, they whiffed a lot more than me, and didn't seem to do anymore per-hit damage. Actually, it seemed pretty variable between us, sometimes I'd hit for 30 more hp than the elvaan drk, other times he'd hit for 30 more hp than me. I missed a lot less, however, so you can guess who was doing more damage overall there.
Concerning the magic side of things, you're better off as a hume than an elvaan or galka drk. Those stat-absorb spells are extremely useful, and galka or elvaan drks just don't have the mp to cast those along with drain, aspir, elemental magic between scythe or greatsword swings. Using absorb spells I've found will lower an enemy's stats by 10 and raise yours by the same amount, which is a very large debuff for the enemy while being a huge buff for you.
Concerning HP, yes, hume drks have less hp than elvaan or galka drks, but you aren't the tank, so it isn't a huge deal really, and humes still get a goodly number of HP. I'm up to about 692 HP now, somewhere in that range.