Vantaggi
If you have long term onsite opportunity then it is very good. You will never get bad rating unless you will be thrown by the customer. If you survived at client site then it is your good luck and it does not depend on how intellectual you are. Mostly people are outsourced at salary discrimination.
Svantaggi
Though have served customer and earned value for them, but once you will be back to offshore location, you will be treated as though as newly joined the company. You will be in stress after as they don't have the project / you will be not fit in the requirement, though you have earned lot credit at the customer end. No body bother how you have provided the value at customer end. You will sitting idle and last you will be thrown out of company.