Which Groupset?

That's a bugger. On my Audax bike I always figured if the rear mech went I could replace it with a MTB one instead.
Also I guess MTB groupsets are better for year round riding?

The materials used in a MTN rear mech are the same as those for a road one at the same price point (roughly).

They are made tougher, designed not to poke out from the bike, and often have cable routing that doesnt require a big loop. This makes them better for riding in situations where they could get snagged. But not that much better for winter road riding.