The problem is that the Flight School sponsored your presence in the US. So if you have a falling out with them or if you decide that you do not like the Flight School, then you are at their mercy and if you chose to cancel your Training, then you would probably have to leave the US within 7 days.
The problem is not the above, but the rumour that the FLIGHT SCHOOL is able to CANCEL your VISA. That is patently not true. It's only USCIS that can cancel your visa. And they will probably only do this if you're in breach of the visa regulations or otherwise misbehave in the eyes of the law.
And to be honest, if you stop your flight training for whatever reason, I don't think that leads to an automatic cancellation of your visa and expulsion out of the country. My guess is that you can still stay in the US until your visa expires. You just can't go to another school to do any form of training (not limited to flight training) and you can't get a job, or do any of the other things for which you require a visa. But you could just hang around and play tourist for a while.
Also, if you were to try to change your Flight School it would probably require a trip back to your home country to sort it out and then reapply for TSA Security Clearnace with the new School.
I've heard stories of flight students who did just that: switch schools, and get all the associated paperwork sorted properly, while in the US. Of course it'll cost a day or two and a few hundred dollars, but apparently it is doable without leaving the country.
two the stories i read on this forum are a real off put. I'd rather take my time and not be screwed over
The school I went to (OFT) did almost exactly as promised (get me my PPL in 21 days - in the end it took 22 days because of the weather) for exactly the price they quoted. Of course at some point in time there will be extras but to be fair, all these extras were my own choice, and the price they charged was for these extras was exactly as advertised. So I definitely do not feel screwed over.
And if I were the exception and the majority of students at those schools would be screwed over for real, then you'd find loads and loads more negative comments about those schools on this forum and elsewhere. And that would eventually mean these schools would go out of business. But instead, the major JAA schools in the US (OFT, EFT, OBA, Naples, AAA) have been turning out mostly happy customers for at least five years now. (That's how long I've been following them.) Sure, there's the occasional gripe and complaint but I estimate it happens no more than with any business. And if you read those stories carefully I always get the feeling that part of the blame could be laid on the student as well.
So don't be put off by the stories. Going to the US to obtain your JAA PPL is a fantastic experience and can be considerably cheaper than elsewhere. But you have to be properly prepared (see my sticky thread on this in this forum) and, indeed, you have to be able to take three weeks vacation. Plus some spare days in case things don't go quite as planned.