Probably equal to McDonald's , KFC , burger king etc .
The oil they mostly use is horrible and that's why I don't eat a lot of street food , as hours later you still get a horrible aftertaste in your mouth . I only use sunflower oil or olive oil if I'm cooking at home .
Obviously some places hygiene standards are non existent and what they put in som tam in some cases smells like a rotting corpse . But otherwise I have never been ill after eating Thai street food , only time I have been sick after eating was from KFC !