Well, this really depends on difficulty setting.
I mean, you can dominate everything with Explorers on Soyuz and below.
If this is Apollo, the only thing that matters is affinity race, and how much forces you manage to build for incoming upgrades before your affinity levels hit.
And since Soldiers and Rangers take less time to build and upgrade first... You figure.
So, on affinity 1-2 which you get real fast, Marines and Gunners are the ones who take it hard in early game, either on defense or offense, also because fortified Marines can stand their ground against enemy Armors, while Gunners rip those Armors, when AI hits affinity 3 before you - and they usually always do. If AI still has rovers this is GG, they cannot stand against you at all then.
So you spam Marines/Gunners, dig those ruins and tech hard, and clench your buttocks until you hit affinity 6, and then your Marines will wipe everything. They will easily conquer you another 1-2 civs who still at affinity 4-5.
Missile units are ways too slow to operate (setup mostly kills it), very slow to react to battlefront changes of running back and forth, and they always lagging 1-2 affinity levels behind when it matters the most.
And this is how they are the reason which basically what lets you to actually slam AI for good as soon as you hit affinity 6 and rank 2 with your marines, they just destroy enemy Armors/Artillery in a blink.
Agree with the dude above, if missile units had +1 range by default, this would really let me think about using them strategically, otherwise just no-no.