Pros
Fast growing company - you can experience and see how one company grow from startup to a real corporate. They have many scopes of business unlike other ad agencies, so you can extend your ability of work if you are willing. Since management has a fresh mind set, your voice will be heard no matter what job title you have, including interns, and you will have an opportunity to challenge things; but you must make efforts and have talent of course.
Cons
Luck of man power as company never stops to grow. That includes luck of mentors for juniors in some teams/departments/offices. Highly recommend to communicate with people from other offices/countries so you are following newest updates and can find the greatest mentor for yourself. At this stage in any startups, employees who just await and do the tasks in front of themselves won’t be able to benefit much from the company comparing to aggressive ones.