Certainly it’s safe, it’s just overload on different muscles through the range of motion, especially if the feet are clipped in to the pedals. The primary loads on the “backside” push-down are hip extension and knee extension, while the loads on the “frontside” pull-up are hip flexion and knee flexion.
Thank you all for your answers. I think I agree with the last answer in that the ability to stop the flywheel’s momentum and reverse if a high risk movement for the average participant. Maybe for more experienced bikers as part of a very slow cooldown if it offered any advantage to working out cramps etc. but not during a spin class itself.
Thank you all.