Eh.
It will WORK your horse, but probably not in the ways you want unless you're riding Saddlebreds for a living. A horse can't move properly in 2'0" of snow, so it's ridiculous and cruel trying to ask them to drop their head and round up. They have to trot like a gaited horse just to get through the snow, and when they canter, they leap frog.
It's a good WORKOUT, but it's probably not going to work any muscles you actually want to be worked. Riding in much lower snow falls can probably be equated to sand, as in, six inches or less, but even then, you have to really watch the footing - the minute you create a track, they can have difficulty with footing as it ices up and creates hard ridges.
I ride all winter but I do not "work" my horses. We play around bareback, we're working on getting the youngsters some basics, but I would never "school" a horse in the snow.
And if you're considering, even for the briefest second, of taking your poor crippled old mare in 2'0" snow banks, I swear to god I will sic Rescue Ink on you.