Not concrete, just soft sand and cement, a neighbour as just had it done, never heard of it myself, whats the chance of it lasting, and why should you not use morter
Mortar is weaker than concrete, and it's more expensive. It's not frighteningly weaker, and for fence posts it;s not the end of the world, but with no coarse aggregate, it's much easier for a mortar to break up or be broken up by external forces.