Pros
A great firm that stands by its values. Amazing ability to work with teams around the world remotely (not purely lip service, it actually happens) and also the long term/ short term exchange program provides unique global opportunities especially compared to competitors. Many of the people you meet are some of the highest achievers in the field but all typically quite approachable and collaborative. A portfolio of high profile projects that makes you quite proud of what you work on. I've worked at a few competitors with major projects and public sector in the same discipline and Arup is by far my favorite culture I've experienced between them all.
Cons
Sometimes workload can be unpredictable (ie. a bit slow or extremely busy) especially for technical specialists, which is somewhat similar across consulting engineering, although to be fair they really work to minimize this. Slow to make decisions/not very nimble - in the past 2 years a lot more red tape has been put up around R&D funds, software & technology, generally getting approvals for initiatives