A well-trained soldier armed with a missile launcher designed to handle both ground and aerial threats, basically a Marine outfitted with a superior set of anti-air rockets that are now anti-armor missiles. They are very effective against Chronite armor and most ground threats, but aren't any stronger against air threats.
Missile Marines carry a missile launcher capable of tearing through Chronite armor. They have additional range over their Marine counterpart, allowing them to kite effectively.
Missile Marines with their powerful and long range anti-Chronite missile launchers are very effective at decimating most Tier-1 and Tier-2 units, primarily Chronite Tanks and Gröditz, easily able to outrange the former.
Missile Marines are similarly effective against air threats as Marines are, requiring similar micro control so be effective. Their increased range also allows them to perform engagements at safer distances, making them harder for Scout swarms to penetrate.
Missile Marines move slowly, making them ineffective at chasing down faster targets such as Snipers, which prove to be a threat to this unit.
Missiles Marines compete for being one of the weakest units in terms of health, bearing a measly 125 health and Light armor. Since they're usually used in groups, splash damage from Chronite Tanks and Missiles are a death sentence, killing them all immediately. Their low health makes them ineffective targets for Meditecs to stand by and heal as well. They perform best when far from their targets rather than up close.
Missile Marines pack a hefty punch in the early game, only struggling against Scouts and Snipers. Therefore, a mix of them with A26-Legionnaires can be effective to help distract fire from Snipers. Scouts will continue to prove a problem due to the cost of these units.
It may prove interesting that this unit is good against the Tier-3 Sakata Spider which features Chronite armor. It may be a cost effective way later in the game to deal with them, and of course take out threats such as Chronite Tanks in the early game. Just be sure to kite, as you barely outrange Chronite Tanks.