Well, it is just a basic military strategy: cut off enemy supply lines, and they won't be able to put up much of a fight. Instead, even without the Crucible, Reapers allowed the fleets to assemble together and hit them hard, and they suffered much bigger losses than they could.
The Reapers have been, well, reaping civilizations, possibly, for billions years. I'm pretty sure they've perfected their craft and wouldn't miss such an obvious opportunity for no reason.
Immunity is something that should have been removed or reworked, IMO. It made playing Soldier, Vanguard and Infiltrator a joke, as level 3 immunity essentially made you immortal for a significant amount of time.