4. If the Reapers fear humanity so much to attack it first and shep in particular for messing up their plans, it would figure that they would nuke the planet from orbit instead of leading a ground invasion. Not even the human husks are an excuse, as they are so much weaker than the other alien species ones (besides the fact that there is plenty of raw material in the colonies).
This doesnt make sense because they want the humans for their next reaper(s) so why destroy the planet with the highest concentration of humans? (the other human colonies cant match whats on Earth.