This just occured to me -- in the Paradox games, you lose men to attrition, which you continuously have to replace with fresh reserves from your manpower pool. Now, I was just reading an article on how unreliable Tiger tanks were, and the thought occured to me -- why not have attrition affect vehicles, too? I figure that the same way you lose men outside of battle, you should also lose vehicles, which have to be replaced. Vehicles can break down, be disabled by landmines, wrecked in accidents, ambushed by partisans, what have you. Areas with high attrition should cause even more of these losses; for example, you should expect more tanks to be lost trying to cross a snowy mountain on poor roads than driving down a highway deep in friendly territory.
I feel that if this was implemented, it'd make terrain types matter even more, making for more strategic depth, and also make the "Reliability" stat feel much more realistic and important, and less like just an abstracted number that helped calculate attack strength. What do you think?