Magic missile is a much better option than fireball and skill is much better than relying on luck
Casting Magic Missile as a level 3 spell will at most do 5d4+5 damage, which on average will equate to 17.5 points of damage.
Fireball on the other hand, with its 8d6 damage, in case of the 4 Githyanki targets will do an average of 112 damage, 28 per target that failed to make a Dexterity saving throw. Granted, they have a 45% to succeed with the saving throw, but that still makes them suffer 50% of the damage, which means they will at least on average take 14 damage, per target. Since I'm targeting 4 Githyanki, there will at least be 56 damage on average, all the while Magic Missile does a fraction of that. So no, factually, it's not a better option at all, in this scenario.
