So I tried out taking a companion with me who uses heavy melee, since my character is a pure mage and dies pretty much instantly to melee attacks.

Problem is, as soon as I use an adept destruction spell which has area of effect damage, or any dragon shout, my companion is instantly screwed and dies even tho he's supposed to be essential.

Is this some kind of feature to [censored] us mages over, or is it a bug I can expect to get fixed?

They're not immortal. If they get attacked while they're downed, they die.

Enemies won't attack them intentionally when they're down, but if they hit them accidentally when aiming for you, or you hit them (like with your AOE), then they die as normal.

Oh, that makes sense I guess.
Thanks for clearing that up, looks like I wont be using a companion any time soon.