I already stated the manufacturer's reasoning for metal expansion/ contraction, was their reasoning for the flats v. the ribs. I'm familiar with physics.
The same phenomenon occurs with nails, except they supposedly bend, instead of break.
What happens with rain in the flats v. the ribs! What is the probability of rain accumulating over a rib v. a flat spot, regardless of screws or nails. Repeat, I installed over shingles, that were and are still weatherproof. Do the holes for nails not cause hole change ... maybe. If the fasteners cause holes to enlarge, which is worse ... the flats or the ribs. No, I did not install purlins (not strapping).
https://www.youtube.com/watch?v=15df...vPoIpRxjCH1Cau
https://www.fabral.com/wp-content/up...2/bluebook.pdf
What is your experience with installing ... that is verifiable.
It's been close to four years, and I'm very happy with the results.