Being able to sneak your guys around is something that seems quite cool, but can be a bit tricky in practice.
This functionality was actually working in-game (entities would become invisible when in close enough range of a tree), but I removed it because it seemed impractical. My feeling is that it works well if you're controlling only a couple of guys, but doesn't really work for a group of soldiers.
Currently, line of sight is reduced by trees, which means it is hard to see objects in the forest anyway. I feel like this is a more natural way of handling stealth. It might take a bit of effort to make AI behave naturally in regard to stealth tactics (using the forests to hide from you and countering these tactics), but I'm currently going with this option. Should men fade/go properly invisible, or should line of sight just be reduced by the forest as it is now?