If it works
Usually there is no need for adjustable cranks. For instance, on a freestyle cycle, you have no need to lengthen the cranks, and even if u did, it would take time to get used to the new length. However, if you used one unicycle for two purposes, and need different crank lengths, than it would be great to have adjustable cranks. If they ride smoothly and are just as durable as any other crank, I’m all for them.
Switching from one crank size to another quickly just takes practice. I could see great advanatages of having cranks that can switch from 110 to 160 or so for long distance.