The dumping is riskless as it happens far from the engine. An engine out on a 4 engine plane constitutes no risk either. But the fact that an engine failed (not an engine being shut down) can imply a great risk. The engine could have damage the wing structure, even potentially rupture the tanks. The dumping may become a risk at it delays the touchdown.
Well SR111 wasted precious time getting rid of excess fuel. I once read that they had a guideline of never ignoring the Maximum Landing Weight but I am not sure if that was the limiting factor there.