When there is a need of hiring resources in bulk to complete specific project or tasks earlier at affordable rates, hiring offshore web design firm can definitely save a considerable amount, hence need to take care of certain things.
When there is an extensive level of quality and complexity in work, you should have ability to evaluate whether hiring offshore firm would turn into good choice as not all the offshore firms can provide good quality and able to deliver complex design.
It becomes necessary to evaluate the ability of web design firm by observing the websites in the portfolio or making casual communication, you don't get trapped in low quality cheap web design firm.
When you are hiring resources in bulk with similar skills, make sure all the resources have equal talent, knowledge and capabilities as final outcome would be combined work of all, each should contribute equally.
The communication gap is the biggest disadvantage due to timing gap between two countries. Make sure they are available at the time you need it or see if you are able to manage as per their given time.
At last, you have to continuously stay in touch an observe the quality of work provided by them to keep your outcome in good rhythm.