This game has mouse acceleration on by default and forces it back on if you disable it. Why any pc game developer would do this is beyond me. To fix this you want to go to %LOCALAPPDATA%\FortniteGame\Saved\Config\WindowsClient and edit GameUserSettings.ini with notepad or some other text editor. You'll want to find "bDisableMouseAcceleration=False" and change it to "bDisableMouseAcceleration=True". After this save the file and then right-click the file, go to properties and make it read-only. Now launch/relaunch the game, figure out your wanted sensitivity again, remove read-only from the file, go back to the game, turn some random setting on/off and apply to update the file, go back to the file, turn mouse acceleration off again, and finally make the file read-only again.
Now if only I could figure out how to change the field of view to something other than a horrible 80 degrees :/
How good is it in single-player? Or does this really need to be multi-player?
The missions scale based on party size. If you place defender NPC's wisely and have a good layout, most missions should be soloable with some effort.