257x257+ maps cause issues with the following;
messy Pathing of almost everything.
Random vision on spots of map.
Causes blight to mess up and even can cause blight strips where a line of black just goes across the map.
Can mess up orders and locations.
Causes the vanilla editor to no longer be able to change stuff like map name,author,description, etc. . . for that part.
Increases the file size of the map by a lot.
Causes load time to be slowed down in-game and in-editor. Same for save-time.
Causes random disconnections and/or desync's. Meaning it can cause server splits meaning different game states players are put into.
I think that's about it, might be missing 1-3 things.
In exchange though you get a lot more playing space.