The new Code of Practice states that mortar alone cannot be used to fix ridges and hips. So, even if mortar is used, the ridges and hips must also be mechanically fixed.
The strength of cured mortar is sensitive to many variables. These include cement or sand type, additives, water content, mortar mix composition, temperature, humidity, cleanliness of surfaces, etc. A simple way of avoiding all the hassle associated with mortar is to use modern dry-fix ridge/hip solutions instead.