i believe medical expiry is important but visa expiry date prevails over any other date.I may be wrong here but as far as I know, their landing has to be before the medicals expire. I remember seeing a post a few months ago where immigration even called the sponsor to determine if the applicants can land before medical date cause the finalization vs medical expiry was only within a month time frame. If I am not mistaken, in that particular case, the parents were not able to depart before medical expiry, visa got revoked and had to go through medical process again.
They always have visa expiry date before which you must enter Canada ( you need decision made before expiry of medcial otherwise you will be asked to redo medical).