You will work hard, and you will mostly not be thanked. While my personal boss was a lovely person, the final say is in the hands of more senior staff. There are some really awful people in high positions who are ungrateful, unprofessional, and unapproachable—the type who people unanimously hate working for or with.
The pay is not competitive with others in the industry. Arup takes advantage of the decision between working for a big name or working for higher pay. Many young staff choose Arup because of opportunity, and then leave to make more money. Sometimes they return later on in their careers when they can make more. The progression is very slow and strategic, which seems to be their idea of "fair" (not an American company).
Work/life balance is not great. Lots of overtime that is not worth your sanity. The engineers really slave away for many many hours. Unless you're in non-marketing central services, you are pretty much expected to work as many hours as it takes.