Even the ones that give decent rates and legitimate currency are just front men for the drugs, people trafficking and prostitution industries using you to launder their illegal earnings into untraceable currency.
In many countries using street money changers is a crminal offence and you can be arrested for using them because the authorities want to crack down on this form of money laundering.
The problem is compounded when you are dealing with a currency you are not familiar with. We probably have all seen Euros or US Dollars but even them we would be hard pressed to spot a fake. When you are faced with ‘used’ notes in a unfamiliar currency that you have very little of in your wallet already to compare like with like then your chances of spotting a forgery is almost zero.
If you are unlucky enough to accept a forged note then you are at greater risk of getting into legal problems if you try and spend it, even unknowingly. If all your travel money ends up being counterfeit then you could be in big trouble and your holiday could be ruined. it is entirely possible that your travel insurance will not cover you if they feel you have been in any way negligent.
You really must check the legal position of using street money changers before you leave and just remember that if the deal is too good to be true then it almost certainly not legitimate. The only real way to get the best exchange rates is to trade in millions of Pounds/Dollars/Euros and have the banks deal with you as an equal. The biggest currency houses can do this but I am not sure how a street corner dealer is going to manage it.