In my opinion mountain supply penalties and defensive bonus are too strong. Two exemples: as discussed here, a campaign in the Caucasus is a waste of ressources for both sides but espcially for the Ottomans. Second exemple, the Balkans; I mean, even when Bulgaria gets in the war early (with diplo incentive), the help of an Ottoman army and the bulk of the Austrian army, it takes years to get rid of Serbia and Montenegro (in my last campaign I finished Montenegro in 1917 only). Historically those two crumbled as soon Bulgaria entered the war. The Italian front is less of a problem it seems. My conclusion; mountain terrain has too much impact on offensive capabilities.