Every once in a while you may experience cramps when using a monofin (depending on your state of training mentally and physically, nutrition, hydration, magnesium intake, etc.). In constant weight a cramp is a rather unpleasant thing but it doesn't have to frighten you off. There is an easy solution to the problem:
--> whenever a cramp is hitting you strong: SWITCH TO IMMERSION LIBRE. Means: pull yourself up on the rope by using arm pulls. 9 out of 10 cramps occur during the ascent phase anyway. The switching to IL works just fine if you use a REAL ROPE. You souldn't worry too much about cramps then. They can spoil a good dive but normally don't bring you in big trouble.
Whenever you are going deep with a mono and a) just do fundiving without any rope or b) use a TINY SMALL CORD (small buoy with a cord from a diving reel for example as orientation and safety) you can forget about using arm pulls and cramps can turn into a real issue again. :rcard
Simply keep in mind what your limits and possibilities (equipment/rope/buddy) are and you should be fine.
Pat