The Breen some how managed to not only reach Earth but attack San Fransisco. If they were so capable of reaching Earth why didn't they simply warp a cloaked asteroid into Earth orbit and have an engine attached to it that pushed it's speed up to close the speed of light and slam it into Earth and wipe out the planets surface?
Why do a regular attack with ships if you can get that close?
EDIT: Infact this is an idea the Xindi in ENT could have used. Why build a thing that huge and have it fire beam weapons? could have just built it and slammed it into Earth at full sublight speed and job's a good un.