I have been on several MSC ships as part of the Military detachments. You have to complete 120 days aboard. You can ask for a relief at your 60 day mark. If you do..... your relief will be there on time(providing they don't become unfit or act of god happens). Delays use to happen as people would go unfit for the ships they didn't want. That shell game has been nipped in the butt for the most part and reliefs are available on time.
Now, if you have a specific qualification, like (Search and rescue swimmer), I have seen delays. However, being in MSC is federal employment. You are at the service of the government and the needs of the country(I.E. the company) and like all federal employees, there are issues.