With the oldest on UK roads at 4 years 5 months I’d hope there’s little evidence of rust so far. It would be interesting though to see an underbody inspection of one used year round in the north of the United Kingdom.
I would suggest that the ND will rust the same as the NC. They look to be very similar in construction to each other under the skin. Perhaps Rodders will chip in as he has possibly the most experience of both NC and ND.
I expect rust will first become apparent where the paint is thinnest; on sharp edges. eg. assembly tool holes on crossmembers etc.
But “rust” on welds might not be what it seems. There is a TSB in that thread (you need to login to read it) that, while technically not applicable to the MX5, might still be relevant (its from 2012, so general principles would cover later models.). The test:
Two original models in 2015, two new ones in 2018 and a 30th Aniversary two or three weeks ago.
On the Mk3 front it appears the really bad ones are from south of the Border where I always thought we used more salt and for a longer duration north of the Glasgow / Edinburgh central belt.