I think the pipework for return lines where there is some pump pressure is better screwed using PTFE tape or glued. A gravity drain line, I wouldn't have thought, would be as critical. I believe those flexible pipes are made from EPDM rubber. As long as the correct diameter pipe is used with them I don't see a real problem. Once tightened, the internal surface of the flexible pipe will form a very secure seal against the exterior of a rigid pipe, i'd have thought.