The problem is that it DOESN'T come back every turn, far from that.
Every 7-8 turns it will decide to not return and you have to fall back to other missiles or take pains to retrieve it, not an easy feat when you're clearing a GUV with it for example.
If you rely solely on the artifact missiles, you will inevitably run into situation where your main weapon is out of picture, what then?
This is probably the cost you're paying for such an amazing weapon - there is a chance of losing it and then you're screwed.
No such thing happens with melee weapons, save for the extremely rare situation of having to clear a ratling duelist vault, and even then it's just one tile to retrieve it and no summons.
When I start using either true aim or thunderstroke, I do so occasionally and I never go against many powerful monsters if I can't also slay them in melee.
Otherwise it would be a big surprise to have to resort to smashing giant kings and titans in a greater vault with a scorched spear or other mediocre weapon, just because you didn't find anything better.
You build your char over true aim or thunderstroke because greater id shows they return when thrown and then *poof*.
In blatant defiance of its advertised properties, it drops somewhere without returning and there goes your build.
You go through swarms of monsters to retrieve it and try to ignore what must have been a rare occurrence but it does it AGAIN just three shots later and gets stomped over by a host of brand new ancient karmic dragon summons.
I had a situation like this with a high-dex early crowned archer clearing a GUV in the jarl cave using True Aim.
Trust me, it's not a funny situation, especially when you had the misfortune of not finding any undead slayers and have to resort to killing emperor liches with 1d6 arrows.
Then you just watch as they heal repeatedly because you're not doing enough damage, while your true aim is lying somewhere in the middle of that rainbow of monsters filling half the level or more.
I will say this: if both projectiles had an undisputed 100% return rate, I'm all in for a 50-60% damage nerf, maybe more.
Otherwise I feel both missiles are balanced.