With seeing a working girl you have more information to determine if she is a good punt or not. I.e confirming services, looking at reviews, taking a look at their profile etc
With agency girls it can be a shot in the dark. The girls have heavy photoshopped pictures, services not listed and you only speak to the maid rather Than the girl. Agencies are good for last minute outcall punts. Finding a good reliable agency is key.
My personal experiences have been slightly more positive than negative although normally gfe is not on the list. I've had plenty of cum dodgers, bad attitude Romanians advertised as italian/russian. Then again i have had good punts. Best to look at reviews.