This seems it would just make it easier for cheaters to post bullets impersonating other players.

Sure, but they already do that with proper cheats bypassing SQF and creating own shot messages. Is it really *that* much worse than creating shots with createVehicle and createVehicleLocal? You can already do a lot of mess with {createVehicleLocal ["HelicopterExploBig", getPos player, [], 0, ""]} forEach allPlayers and its not even logged in createvehicle.log. (Censor this if needed)

Amount of modding benefits greatly overweight small amount of potential harm (which I'd argue is none as cheaters can already do all that)

Reposting from discord:

I think the main use for this command will be clientside for scripted weapons and the like, forcing it to only be run on the server would add lag to the mix and be bad for general 'feel' and hit registration locally.

If security is a concern the command could have two different behaviours when running on the server or a non-server client in MP.
For the server the command is unlimited, any unit can be assigned as INSTIGATOR/VEHICLE
For non-server clients only units local to their machine can be assigned, so their player unit, AI local to their machine etc. Or objNull ofcourse