Just from a personal perspective, I've always viewed the Reapers as a cheap imitation of the Shivans. Same basic villain archetype with all the mystery and ominous removed or campified.
Also from a firepower perspective, Shivans wipe the floor. Reapers have to descend like locusts onto one world to slowly tear it down, whereas a Shivan superdestroyer + destroyer team managed to completely glass a planet and destroy most fleeing vessels inside of 13 hours. That was just two capital ships, and we have yet to see what would happen if a Sathanas turned its guns planetside.