1. Concern that people will send out virsuses disguised as the game's map files (which are text files).
Yea I thought this too , but the chances of this happening is very unlikely. Has never happened in the 4 online RTS games ive played - TA Spring , Supcom , Homeworld 2 , Empire at War ,
2. Concern that some cretins will ruin other people's mulitplayer experiences by making (intentionally) imbalanced maps.
Yea this is true too ,but ive observed in sins that people wont even join a custom map hosted by a random person , because people have the mentality to only join maps they recognise. Since I talk alot about my maps in the lobby and people do know about me and my unhealthy obssession with mapmaking

, I can stir up enough curiosity to get people to join my maps . Thus an idiot who tries to host his own IMBA map ,isnt going to get many people joining unless he puts in the effort which isnt worth it . In Homeworld 2 , it was very rare to find people making imba maps. Only one idiot called Dicen-TLSC hosted imba maps.