Its as safe as anywhere in central Brighton.
Usual common sense applies - dont walk around on your own in the dead of night or when drunk, and use door/window locks. All student flats are an easy target - they have lots of nice goodies like laptops, bicycles etc and most students dont lock things properly or have insurance. Coombe Road itself is fine. Its close to Lewes Road which has great bus routes to almost anywhere and is walkable into town.