One other thing is that the more hydraulic, the harder that is generally, the harder your materials being cemented together need to be. Using modern bag mortar with old soft brick or even a soft enough stone will, if exposed to the elements, cause a deterioration of the softer stone or brick. The softer mortar made from an air cure lime will take the brunt of any expansion & contraction in the wall, while a harder hydraulic mortar will attack the material it's holding in place.