Is cubital tunnel syndrome rare?

No. it is not rare but less prevalent than carpal tunnel syndrome for every 20 carpal tunnels i do one cubital tunnel release.
Fairly common. Cubital tunnel problems can occur fro many reasons and are fairly common. Most causes can be treated conservatively and it is worth taking the time with a doctor and physical therapist that will work with your to correct the underlying problem. Surgery is a last resort and is usually effective when other methods don't work.