2d4 per caster level is not all that much compare to 4d6, 8d6, 12d6 for 3.
That doesn't balance well?
I suppose I could make it a ranged touch ignoring amor bonuses. Force effects are not going to be deflected by a physical barrier, otherwise magic missile could actually miss someone in full plate.
The sword is not the only such spell that ignores armor, however, dodge, dex, deflection and other defenses still apply. Which is not the case for all or even close to all spells. I can give you a similar scale example if you would like. Also, the blade itself is made of force, the ranged attack made using the spells effect is not.