Not the most exciting place on earth. Nielsen is old school however you look at it. Great to start your career, but there is a very rough patch to traverse between Sr analyst roles and management ones: fewer opportunities open, very competitive, not easy to get the skills needed to move forward (as they are two different types of role). So progression is a question mark: some people I've seen rise seamlessly and very fast, others to slug forever in the same role (and not because they were bad employees!)
Oxford offices are not conveniently located.
Company can be somewhat corporate and callous to their employees, you get that "is down to HR my hands are tied" kind of talk.
Also, very siloed. If your team is small and you work from home/client often, you will not make many friends at work.
Everything needs to go through 100 miles of red tape before any decision is made.