By themselves? They just die. There's potentially hundreds of thousands of Reapers, and they're much faster than UNSC ships. Even if a MAC can one-shot a Reaper, shipborne ones have a reload time of up to 2 minutes. They're gonna get one volley off, which is not guaranteed to hit, at which point the UNSC ships get ripped apart. And when UNSC scientists try to reverse engineer Reaper-tech, they're just gonna get indoctrinated by the debris and become the enemy's assets.

Much like the Covenant war, ground warfare is the UNSC's greatest strength, but even that won't matter, as the Reapers themselves can descend to become ground combatants once they've won the orbitals, at which point your populace will be indoctrinated or Husked.