If you are playing by the single-roll interpretation, the added damage from spell bombardment gets added to every dart. The average rises from 3.5 to 4.125. If you use empowered spell to reroll 1s, you push the average dart damage to 4.66. if you reroll 1s and 2s, 4.94. If you reroll 1s, 2s, and 3s, 4.97.
I just roughed those numbers out, so they could be inaccurate, but the point is that spell bombardment interacts in a weird (and beneficial) way with magic missile, such that it competes with scorching ray (without greater invisibility) and lightning bolt in low- and mid-level slots. Against high-level enemies with legendary resistance and/or spell reflection, magic missile can be the best option.