125mm on a 29" would be a problem for me. Like @Bug72, I am approaching 53. Lots of hills and uneven terrain in my neighborhood. Shorter cranks may require sudden, strenuous corrections, putting stress on the ankles, knees and hips. Longer cranks seem to exercise a larger range of motion. If you’re accustomed to cruising on short cranks, then switching to longer cranks will feel like slogging. Not ideal, but perhaps better than injury.
All aspects of unicycling are interrelated. When I learned to ride with both hands on the handlebars, I was able to spin bigger, smoother circles, the feeling of the dead zone at the bottom of the cranks diminished, I no longer felt that long cranks were throwing me off balance with each pedal stroke, and (most importantly to this thread) my hips started to rise and fall more naturally during each pedal stroke, adding to the effective length of my pedal stroke and keeping my hips from being frozen in place. And, riding two-handed moves the balance from one or both outstretched hands to the hips, increasing hip mobility.