Yeah, some factions get a massive bonus fighting in forests. Most notably Lusotannan, they can get up to +4 moral while some enemies can get -2 or more?. That's a 6 point swing (for the mathematically challenged) and makes for an easy win against the hated Carthies or Romans.

Plus it really makes cavalry and missile troops less powerful, so you cant use the same tactics you use on open ground. Overall, forest battles are annoying but ignoring them does cut a big chunk out of the game IMO...