Clients can be difficult to deal with.
Once your project is over you will have to go through interview process for a new one, it is a lot of pressure like being on the job market.
Work life balance is horrible, especially when working with US clients. You are expected to work 9:00 to 18:00, but also attend any meetings after 18:00.
Career progression exams are iffy and you don't have time to study or apply to them when you are in a demanding project.