Yes it is. Our ships can only go 50 light years before having to discharge. We have no idea how the Reapers surpassed this limitation, so we'd still have it if the voyage left before or during the Reaper War. And since Andromeda is a lot further than 50 light years away, going there is a problem. If that problem is suddenly solved, it is a DEM.
It would need to be a ship or ships that can make the trip without killing the crew for hundreds of years, carry the hundreds or thousands of people that are going on the mission, carry all the necessities like water and food for both the trip and the time it takes to find a place and cultivate it, a fuel tank that has enough fuel to get us there, a powerful Mass Effect drive to help us get there, tools to build once we get there, weapons and vehicles to defend in case of attack by hostiles, and so on. That project is massive, no matter how you slice it. Too large to hide before the Reapers arrive, and too large to do after they've arrived and all top minds and resources are going to the Crucible. That's not baseless assumption, that's logical deduction.
Not to mention the fact that during this period the galactic economy would be under severe stress, and many supply lines would be broken.
I could just about accept the project beginning around the start of ME2, if the Council were only playing dumb with their "Ah yes, Reapers" routine and were in fact frantically laying contingency plans that they hid from Shepard, for some reason. I wouldn't much like it though, and it would strain my credulity almost as much as the Lazarus project.
But as to an ark being thrown together during the Reaper invasion itself... No. My supplies of disbelief suspendium would run out at that point.