Vantaggi
Very few. Mostly the relationships you can make with very interesting and talented people. Some cool projects, if you can muster enough support from management to pursue work from clients.
Svantaggi
Clueless management and byzantine organizational structure that is always in flux. No local management with decision-making authority, even in fairly large offices (75+). No business development support (in terms of dollars and proposal help) for technical staff on the front lines who deal with clients day-to-day. Avalanches of idiotic communications from senior management. A medieval employee rating philosophy and scheme that force-fits people into a normal distribution. (But wait! I thought AECOM employs only the best.) Relentless efforts to direct-bill clients for what are actually overhead functions. This is an engineering company run by accountants and MBAs who have starry-eyed visions of glory, but who apparently run the business by sleight of hand.