Terrain does have a big effect on range but as long as they all are rated the same way it is useful for comparing the various antennas. IE: a 60 mile rated one will get 2x better signal than a 30 mile one.
It's not just terrain , the transmitter power , altitude as well as the antenna's altitude effect it too.
It's kind of like mpg ratings on a car...............YMMV skuh kuh kuh kuh