I think that you should try and use a level up system for this game. Increasing a single units survivability on missions(making a scavengers ability to find more increase, or soldiers ability to survive/increase defense of the fort etc). This would make it easier on the higher difficulties.
Also I think Science should do more. Like being able to construct defenses for the fort(Cannons, Mortars and other stuff). Then you have to scavenge for the parts and finally have your units construct the defense until it malfunctions or something. This would make the game more interactive and more possible at the higher difficulties. Still a solid 5/5 though.
oddly, I was playing on my computer with networking/safe mode = my computer is so slow now a days that stream was.. in bullet-time ? It was so slow.. I got the impossible without this, but if you need help I'd try this if you can react in time :)
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