The blades of a rigid rotor system are not any more highly stressed than the blades of a conventional rotor are, assuming both are made of similar materials. Either type of blade is designed to accommodate the load/lifecycle required from the blade given the properties of the materials used to construct it. A rigid rotor blade is obviously much stiffer in flap bending and torsion than a conventional rotor blade. Thus a rigid rotor blade experiences less bending and torsional deflection than a hinged blade does. But the relative stress/strain of each blade design would be similar.
The much stiffer rigid rotor blades also create far greater out-of-plane (cyclic) moments about the mast. But the trade-off is that the rigid rotor is much more responsive. The reason rigid rotors have not been more widely used is that it is incredibly difficult to design a rotor hub/mast system capable of handling the huge out-of-plane moments rigid rotor blades produce.