@veteranashe but isn't that the same for a corsair that deals 1200 damage? It is quite difficult to stop 1 or 2 corsairs, they are faster, have more hitpoints (1100) and their weapon range is greater with 40 vs 25.
This 1 Mercy will get more damage from first pass, but if you get 3 passes out of a corsair it is already more mass-efficient (not 2, as it costs more mass).