There are quite problem's with that, first there is indoctrination, exposure to any reaper artifact for a prolonged amount of time results in becoming a reaper servant, there is no way anyone would know what the reapers are without going to reaper artifacts capable of indoctrination and even then they have no idea what they are or what they are capable of. There's also the fact in evolution comic's Saren and TIM are the only ones who knew about the nature of the reaper artifact and given who they are I seriously doubt they tell any one about it. Three you'd think if they knew what the reaper's were they'd tried to warn the galaxy about so they can prepare their fleets/armies for their arrival rather than just go off on some secret project that no one knows about, that is doomed to fail that will only save a select number of species etc.. there are just too many issue's with the theory that makes unbelievable and unfeasible.
Who said anything about prolonged exposure? I'm just talking about finding out what's what. Just because Saren and the Illusive Man were the only ones in the comic doesn't mean there couldn't have been others, other artifacts, other groups that managed to discover the truth without being indoctrinated. Then again perhaps their source was Prothean, not Reapers, and they simply reacted differently than Shepard.
Perhaps they simply concluded that if the Protheans couldn't stop the Reapers, they didn't stand a chance? Perhaps they decided to save a few rather than tipping their hand to any potential Reaper agents by going public. Or maybe they just knew nobody would believe them.
All of this is quite feasible.