Longbows are not all powerful. They have limited line of sight and on top of that even a longbow on a hill can only fire over neighboring terrain. The terrain 2 tiles away can block LOS which means unless it is 2 tiles of desert, plains, or grasslands, the longbow can't easily get a shot off.
Forest hill is the only terrain can block line of sight, even hill or forest alone can't block longbow on a hill. Also longbow on a hill means completely full line of sight, if that's not powerful, I dont know what is powerful.
Edit: about the line of sight, I don't know if hills 2 tile away will block LoS, I remember it does not, but a melee or mounted scout can be put in front for sight.