Building next to ours had the foam treatment about 15 years ago. I
sucked my teeth at the time but it appears I was wrong.

A friend also had foam on the back of unfelted slates. That seems to
have been good for many years.

However 15 years isn't long in the lifetime of a roof - and usually
there is some hope both of reusing roof coverings and that the rafters
will not need attention - neither may be possible when a foam treated
roof finally does need attention.

Another factor is professionalism. I would suspect that a foam
treatment could be used to patch up a failing roof without fixing the
real problems - rain leakage may no longer be seen - but rafter damage
may be occurring.