Total Death and Destruction
I'm not saying it's advisable, but Reaper can be exploited.
Reaper = population death of half the reaper value.
Therefore, Reaper 10 = 5% pop death.
Therefore, 20x harvesters of sorrow (each with Reaper 10) = 100% of the target tile's population death.
Because of scouting and turn order, reaper effects will take before any patrols take them.
This can be mitigated by the following:
Use reapers when the PD of a tile is already destroyed: when you or another party is sieging a fort.
Alternatively, you can use Reapers or BVC-zotz/caelites (less effective) to hit a fort anyone owns with the idea of diseasing the siegers (with the side effect of, well, reducing the population of the tile, as well as the corresponding supply)…
1 reaper = 10% of siegers diseased, with the only defender's option to magic-phase attack your reapers. Not a good option!
10 reapers= 100% of siegers diseased. 
Sincerely,
Bob (the Destroyer), from Discord 
also known as sob on gta5mods forums