I don;'t think it's that AW sabotages the girls as such, more that anybody can advertise there and there is no realistic form of feedback.
As such you get so many scammers, bait & switch and bad services - but it all depends on the girl. Girls who provide good services have to advertise there too - and then it becomes a case of filtering through them to find the good ones. Thats where UKP can help either explicitly or just knowing what sort of things to look for.
Agencies - I guess everyone said it already. You just pay 50% more and there's no guarantee of quality, that the pictures will be authentic or anything else.
I guess the general concensus would be stick to AW really, and if you get a bad punt it's cost you £100, not £150, and if you want a virtually guaranteed good punt stick to a girl with multiple reviews here.