It seems that there are many non serious schools.
What do you mean by non serious flying schools ?
There are plenty of excellent smaller schools around in Florida and the UK (I can't speak for SA) that provice but don't necessarily specialise in commercial training because they cater to broader spectrum (ie PPLs, charter and rental) but this doesn't necessarily mean the quality of service is any worse, infact you may even find them to provide a better service with more instructor time, better aircraft availability, more flexible hours of opperation and a more personal level of service. You need to weigh stuff like that up when it comes to choosing between a small organisation and a 'pilot factory'.
On pilot factory style schools,I occasionally notice Ormand Beach taking a knocking on this forum so in case you are considering them and to balance things out, I have 2 good friends who are now flying for major UK airlines who went to Ormand beach. I've spoken to them very recently about it, they had a few minor gripes after they finished there which are long forgotten but they'll tell you on the whole that it does exactly what it says on the tin and more importantly both have said they'd choose them again. I also know 3 instructors who used to teach there, none of them had any major problems with the outfit, all returned because they had jobs back in the UK, 2 are now retiring, one is flying jets.
Paying much more than expected for example?
You will often pay more than quoted unless you are very talented and very lucky. Prices quoted are for minimum hours required by the aviation authority concerned. It's not unusual to require more than minimum hours, fuel prices vary and often the cost quoted is based on the fuel price at the time. If you fail the odd exam then the resit may mean extra money and you may even fail the flight test, it happens a lot and may require extra flight time and another test fee, not forgetting the cost of renting the aircraft while on test. Other sundries can and usually will crop up. It's nothing to do with being ripped off it's just the nature of the industry.