If you have the wrong canter lead it can be hard for the horse to keep its balance. The can be escpecialy hard when jumping, and getting the right distance to the jump. It also tends to look nicer and feel nicer when on the right lead.
Having the "wrong" canter lead isn't bad nessicarily. You'll have to pick up a "counter canter" in select breed shoes and eq classes to demonstrate that you can be versatile with leads and maintaining them. However, being on the "wrong" lead during a canter can feel bumpy or disjointed simply because of the leading leg being on the opposite side of an "inside bend". To my knowledge, a lower skilled rider may cause the horse discomfort from impact of repeated bouncing and may throw the horse and rider off balance during the opposite canter lead. Hope that helps!