I hope im not copying someone elses ideas.
I believe being located in tundra generally has little or no upside, but i do think there should be an advantage to it. For example, what if (as somebody suggested at the start of the thread) units that end their turn on tundra or ice would loose a amount of hitpoints unless they have a promotion that renders them immune to it, and maybe units that are built in cities with tundra or ice would recieve this promotion automatically because they are indigenous to it.
People that grew up with cold, long and harsh winters have an edge over people who didnt. And soldiers who recieved their training in tough arctic conditions subsequently have an edge over people who dont.
Historic examples are: the french invasion of russia, the german invasion of russia or the russian invasion of finland.
That means that if you are a southern nation who wants to invade a nothern nation, you need to build an army separte for that, or give your existing army promotions that help them withstand cold weather conditions.
In "alive" the survovors of a plane crash in the snow mountains of the andees (i think it was in south america) resorted to eating their friends who had either frozen to death or died of hunger.
Arctic conditions requres a different set of equipement (tents that can be heated, which includes firewood) and several layers of clothing to regulate core temperature and avoid hypothermia. Food is scarce but walking through snow burns large amount of calories.
Ofcourse, with proper equipement and corresponding survival tchniques it can be done, and has been done.
The winter war between soviet union and finland had a death ratio of one finn for every ten russians. This was mostly due to bad equipement, poor leadership and some morale issues on the soviet side.
I guess scouts can handle themselves in most conditions.