Balancing is not easy, settings are static and cannot change dynamically during a game. BF2 bots are totally dumb, they have 0 tactics, and their only defence is their accuracy. Having easy settings is fine for low population, but way to easy for medium and full population. And on the flip side, having hard settings still gives a challenge for a full server, but low populations have an impossible task without teamwork. There is no "perfect balance" PR is all about the teamwork, and using that there shouldn't be any problem completing any map. (they are only dumb bots, and your squad has 8 brains)