There is no flexibility in the work schedule but you get better hours with tenure. You have little say in what work you will do, but that is actually a positive because if you are doing well, you will get the opportunity to move around and get a well rounded view of the organization.
It is made very clear in the interview process that if you don't come in as a management candidate, it will be a very long time (like 5 years or more) before you will even be able to think about moving into a supervisor role. They aren't as clear however that you may never be considered for roles higher than supervisor. The positive side is that McMaster doesn't care about titles so even if you don't get moved to supervisor, you still can get raises and promotions for moving to different departments and adding more responsibility to your plate.