Ya know, I try not to worry about that sort of thing.

Except for a couple of rather obscure specs and standards that I assiduously ignore I could care less whether someone in a marketing department decided that I have "road" cranks and an "MTB" cassette.

I say just figure out the gearing that suits your touring style, and use whatever compatible components will achieve that gearing. I see no particular reason to worry about anything else.

As to tougher, it seems to me you can get crap MTB and very rugged road parts. Shimano also makes a handful of very expensive high-end lightweight parts -- both road and MTB. I wouldn't use that on a touring bike though, too pricey and too likely to sacrifice a tiny bit of robustness in favor of reducing weight.