I think bribing is a little overpowered though. For example: In the campaign as the brutii you can basically get so rich you don't have to fight a whole lot anymore.

That's the reason I try to resist the bribing temptation. In my previous campaign I did bribe and I ended up only fighting sieges and none of the normal battles.

And fighting the units is sometimes cheaper then bribing. Consider this. A few 100 bucks for bribing a unit consisting (for example) of 5 spearmen/ against taking a few horsemen (which you probably have laying around doing nothing somewhere) outflanking them and then destroying them.

It gets you experience, the unit gets experience, you'll be so much more satisfied