The main reason I'd not want to do it in winter is the possibility of getting hit by a blizzard while crossing Northern Ontario which is basically unpopulated, and there are stretches without cell coverage. But that's a risk whether you have a gas or an electric car.
One rule for winter driving is to never let your tank go below 50% so you have enough for heat if you get disabled. Heating the cabin in an electric car is really tough on batteries, but you should be able to survive just on seat heating in an emergency, so half a battery should last a long time. Tesla has enough chargers to let you stay above 50% on a cross-Canada trip, but you're going to be spending a lot of time charging. You'll be charging more often, and that last 20% is really slow.
The normal advice is to take I95 instead of the Trans-Canada in the winter, but with borders closed because of Covid, that's not currently an option.
Also, wile a large percentage of the population of Canada lives in Toronto and Montreal regions, where cities are closer together, this does not mean that is 95% of the population. Calgary alone has a population of more than 1.3 million people. It is very common to need to make a short trip to Edmonton [0], which is considered close by Canadian standards. But even then, it is a 600 km round-trip in a road that tends to be snowy quite often. Population distribution in Western Canada is much more spread out than the East. And the cumulative population is way more than 5%.
[0] For example for concerts. Calgary and Edmonton are big enough to get good musical acts. But they are not big or far away enough for touring musicians to visit both cities. If You live in Calgary and your favourite musician is having their concert in Edmonton, that's a day trip you will most likely take.