Ah, but Matt Risinger did use Spray Foam in that house in some places.
It wouldn't surpise me if Rocwool sponsored the video and he's gets his labor at cost which he doesn't mention. Someone else may use more SF due savings on installation.
The important part is knowing that with ever material there are places it should be, can be and shouldn't be used.
The best application I've see of SF in a roof was it being sprayed over 1" EPS. The EPS was spaced down to allow venting/drying and drainage if the roof leaked.
The ideal solution is to have a continous layer of insulation on the outside of the structure.