Vantaggi
Big company and ability to branch out and work on different types of projects. Make sure you do your research on the actual team you're working with, like any other place! And don't count on the good or bad name of Stantec. Because they have acquired so many firms that the groups you're working with won't really be Stantec! And every group is going to be very different.
Svantaggi
Depending on which team and which geographic region you work with, there seems to be no oversight, and rather only hinderance from head offices, especially if you are part of a recently acquired firm. There is the opportunity to grow to some extent, but it all and solely depends on your manager and his/her managers, and your luck with them. There is not a well established system for that (only words). Companies that get acquired get neglected and the employees of those firms get discouraged.