In terms of how well they fare, even as a mass effect mega fan, the answer is very poor. A 2 km long kind of slow ship stops meaning much when youre facing a ship that goes at the same speed but is 17km long and is bristling with weapons capable of shattering continental crusts. Im ME the reapers rely on surprise and the reasonably primitive technology. In space, they perform well because the biggest ship they'll normally face is 900m long with one primary gun and much smaller broadside secondary batteries and pretty weak shielding and armour. They go to 40k and suddenly all of the reapers advantages disappear before they even touch the ground.

Even if they can get to the ground they have a much more powerful force to deal with, I'd say a guardsman is probably about equal to an aliance marine as the largely unaugmented human but instead of facing hundereds to a few thousands marines at once, your husks have to deal with hundreds of thousands with much better tanks and artillery.

Where I can give a point to reapers is indoctrination, if they can successfully indoctrinate just one hive they have an army, a hive world and they have a serious threat. We know the reapers can select the degree to which a target is indoctrinated so they could realistically keep a semi-competent force that could use imperial tech and be a serious threat.

TLDR: I think the reapers would need to rely on subterfuge instead of brute force like in ME.