They can leave you helpless at any point of time, even though they say that you are a permanent employee, as long as they have profit from you they will keep you and afterwards, they just don't care!!!
So you will get salary of permanent employee (Less) and you will always be at risk of contract job!
In the begging they will commit so many things, so many promises like you will get full salary even on bench, you will never be out of a project etc, etc , but all will go in vain if there occurs any issue with client / project, even if issue is at their end.