Met with a Romanian girl before I was on this site. Can't remember her name, but very average meet.
To answer your question, common complaints seem to be
1. Not the girl in the pictures
2. More restrictions than advertised, or unusual restrictions e.g. no kissing. This isn't to say that the escort isn't allowed to have rules - the problem seems to be that they're unpredictable.
3. Negative links with suspicious-looking pimps
4. General sense of lacklustre "performance"
Those are big generalisations, but hey, you asked.
There are probably others.