Assassins can be support demigods.. or so can Generals.
It's just Assassins only control their one unit. Generals can build additional ones (angels, minotaurs, catapultasaurus, maybe more plus 1 unique unit per general IIRC) and control big armys like a normal RTS.
But just because they're called ASsassins.. it's just the distinction of it being one person. The Assassin could, I assume, be a healer. It could do big heals to one unit, do aoe heals, chain heals, or something. I know of no such Assassin, but that kind of unit would be one.