Actually
I'm most worried about muck, a common obstacle in summertime northern Canada.
Whether there are soggy places an ATV could paddle or float over with its
broader footprint that a horse might get stuck in.
Is there anywhere an ATV can go that a horse could not? There
are some tracks and "winter roads" in northern Alberta that are described
as "ATV-only" that might be lovely to ride. I don't mean legally
rideable, I mean practically and technically rideable.