I think it depends on escorts capacity of managing her income. Also the money she is spending.
It depends on personality as well... if the escort likes others to control, while she just does a minimum effort. Or the escorts likes to control everything, and she does it better than being with an agency.
Having a good background, a good reputation, some regular clients, helps you survive independent and be able to pay adds/rents/etc... and makes things easier.
Language is an obstacle for many girls. Communication is a plus in any job. I think that is the main reason for many to keep working with agencies.
In other countries, like Netherlands, most of the escorts are working with agencies, because they are legal. They work better together. But in Ireland the idea of agency is the equivalent with pimping and trafficking.
Actually in other countries the agencies are making a casting for the escorts working for them, but they also offer a good quality of clients. And they are promoting the girls in a real way.