Not sure if you mean Debug Package Enabler, or Debug Settings.Why people say that the debug is dangerous?
Debug Package Enabler: Its not dangerous, its just that HAN enabler and signed pkgs will work all the time (if you do everything right), Debug PKG enabler will only work sometimes, so its best practice to just use signed pkgs with HAN enabler, but if you know what you are doing, you can get away with using unsigned Debug pkgs for PS3 disk games that have no NPDRM, this will save you having to resign them.
The reason people warn against using Debug is because noobs use it when they don't know what they are doing, and then complain their games wont work.
Debug Settings: Not dangerous, but does not have much use for most people and has downsides like no psn access.