Vantaggi
In general, Stantec's message to new acquisitions is, "If you're doing something that works well, don't change it." If you are in a position below the Project Manager level, this is largely true. Your company was acquired for what it is already doing well, and in general Stantec does not step in to change your project delivery processes. Upon my company's acquisition, there was not an immediate round of layoffs and/or house-cleaning. (although it eventually happened to some people...) Like any company, there are plenty of great people to work with if you seek them out.
Svantaggi
Stantec is singularly focused on acquisitions and how to plug new companies into their existing framework - there is little to no focus on organic growth and/or how to improve the way things are done. They are not a known entity in the Buildings industry, so you'll spend a lot of time explaining to clients who Stantec is and what they do. If just acquired, you are about to become part of a "Business Center" (formerly called a "Profit Center"), in which Oracle data is king. For any Project Manager or above who might be reading this, your world is about to get turned upside down. For everyone else, you will mostly just experience a laser focus on your utilization rate and completing timesheets on time, due to the manner in which Stantec recognizes revenue/profit. IT and HR support will become more impersonal and less helpful, and your local IT/HR support might be replaced by remote support. Your internet speed will probably get slower and web browsing will become restricted (so long, Youtube software tutorials). In my experience, Stantec noticeably lagged the DC market in terms of salaries, and the benefits were not as good as I'd expect from such a large company. Bonuses occur at an odd time of year relative to other firms and are tied to the performance of the entire corporation, not just the success of your projects, office, or even your whole business line.