[Taqwus]

You could see an impact on starting distance or initiative -- ex. an army with more units with forest survival being able to get initiative even on the attack when ambushing enemies in a forest.

Survival skills and terrain might also affect stealth/patrol efficiency and ease of moving supplies from a nearby fortress.

On a more universal level, you would expect a forest to hamper ranged attacks (lots of cover, difficulty spotting) so a precision penalty would make sense, and possibly attack penalty. It being a forest might also force a shorter initial distance between the armies.

In mountains, one realistic but potentially complicated and dangerous factor would be making it difficult to coordinate multi-province attacks. Mountain passes might also have closer initial deployment; and one might want to see encumberance penalties.

Swamps are going to be brutal on ground-pounders; heavy encumberance, probably lower att and def as well. Patrols should have a harder time finding sneakers that have swamp survival.