Earlier quoted context omitted.
Water _will_ get through your roof at some point, and it'll sit nice and tight right above your spray foam, for years, decades, and you'll never ever know about it until the problem is too big to handle. This is especially true for hack jobs on older buildings.
Whereas the traditional warning sign in older UK buildings is for you to know about a problem with your roof by having your ceiling plaster collapse when you least expect it!
A lot of people spray foam in old attics as a set and forget insulation measure: https://www.stellrr.com/wp-content/uploads/2021/04/AAF45418-...