BioWare was rather wishy washy on the Reapers' nature; at one point they are the epitome of Lovecraftian horror, with the whole "Even dead Gods can still dream." and the next being a conventinal foe, the 'Reaper Vulnerabilities' codex entry; with understandable motivations, ME 3's endings.
The major problem (IMO) was that BioWare wanted it both ways, they wanted the utterly alien nature of a creature out of a Lovecraft novel, while at the same time being completely understandable. Why do the Reapers harvest? Who made them? Where do they come from? Answering these questions was at odds to how they were presented, and ultimately lead to the rather lackluster Catalyst and his cycles.
Knowing their origins was not the reason they fumbled at the end, it was describing something that was presented as unknowable in simple, easy to understand terms. The Reapers' reason for doing what they do, that should have remained in the dark.
Lovecraft horror and by extenstion the Reapers, rely on staying in the shadows of the unknown, shining the light of logic and human reasoning on them reveals a shoddy monster costume that instantly deflates our sense of awe and/or terror at the prospect of them. There is nothing more scary then a person's imaginagtion; we can think of something far more terrifying in our own sub-concious minds then what can ever be shown to us.
The
Suul'ka of the Sword of Stars universe handle this notion better then the Reapers did, again IMO.