Why is cement Render bad?
Cement renders traps the moisture, which through the action of frost and the concentration of salts accelerates the decay of the stone.
Cement-rich mortars, as these will set harder than the stone. Cement mortar is less porous than stone, so moisture gets trapped in the stone and evaporates at the stone face, causing it to weather rapidly and over time leaves the render breaking away from the building
In addition to damaging the stone stonework, cement, rich mortar looks out of place, as it is often a gray or off-white colour which does not complement the colour of the stone.