Ventajas
The pay was greater than the full-timers and were jealous. Improving client code to modern, testable standards.
Desventajas
Typical consultancy hire you based on excellent skills required to win over a client - "wow this person is great". Then lead you to a dead end contract whereby you don't gain any skills, it's actually a downgrade old code most the time as it's client based. The full timers leak reports back to management, keeping tabs but you never hear from them until you get the tap. The tap always comes for contractors.
Full-timers code is seriously questioned. They work fast like a whipped horses so they're conditioned but produces horrible code bases and issues. You just learn to keep quiet and NEVER criticize - it gets leaked to management.