Vantaggi
As part of a large company, you may stand a better chance of not being booted off of a government contract. Maybe.
Svantaggi
All electronic systems, including payroll, mandatory training, benefits enrollment, etc. are a technical mess and a nightmare to navigate. Every website related to CGI is visually and functionally flawed. This is astounding for what is supposed to be a technology company. The disconnectedness is evident in every operation. For example, I was ordered to create multiple resumes and descriptions of my KSAs for multiple purposes: performance reviews, specific statements of capabilities for contract bidding, human capital inventory (theoretically), you name it. But none of these systems talk to each other. CGI has no idea who is working for them and what they can do. because they have no means (or interest, apparently) in capturing and using this data. If they need something done, they hire somebody. If they needing something they let you go. Bad business.