I am not a construction expert, so check your sources on this.

My understanding is that, for new construction, spray foam is most often used in areas that aren't likely going to be damaged by condensation, such as against concrete, or metal. I had looked into spray foam for my home, because my home was built in the early 80s--before building codes existed in my area--and there's no cladding on the house and just fiberglass bats between the studs. Because there's no cladding, there's much more air incursion through the bats, esp. since the interior walls are lapped wood paneling rather than wall board. Spray foam would have been a total air barrier, but it would end up being applied directly to the inside of the exterior siding, which would be a nightmare when siding needs to be replaced, and would probably cause moisture issues.

The best solution appears to be to use 3" EPS foam cut to fit between the studs, and then use spray foam to fill any gap between the studs and the EPS. That still allows an air gap between the foam and the exterior siding so that moisture can evaporate.

In the case that's being cited here, I'm not sure why they opted for spray foam over EPS or fiberglass bats. If their home is well sealed, then bats should have been perfectly sufficient, although they have a lower R-value per inch than EPS. Oh, and the difference between polyisocyanurate and EPS/XPS is about R1.5/inch, but that difference drops to about R.25/inch after a decade. That meanst that you don't gain much in the long term when you use faced polyisocyanurate board. I'm not sure what blowing agents are used for polyiso; it might be more environmentally friendly to manufacture.