Indeed, just because a flight school is foreign does not mean it is going to be rubbish, I have dealt with some pretty ropey UK based schools - but you learn and move on!
If you are going to drop a large amount of cash into a school then do your homework and visit if you can. if the school is abroad and a visit is out of the question then ask ex-students, use a board such as Pprune to ask around, quiz the school itself. And never pay up front!!!!!!
Using the word 'cheap' in the sense its being portrayed along with 'rubbish' is misleading - the reason foreign flight schools, such as the US, are good value at the moment is that the £ is pretty strong at the moment, exceptionally strong against the $.
J.