The key "complaints" of GP, as I understand them, are that being simultaneously the victim and prosecutor, the prosecutor-as-victim is more incentivized to use heavy handed tactics during the prosecution process.
Whereas a generic prosecutor has a bunch of cases of reports from victims that are not related to them, and thus if a case is not sufficiently strong, they'd normally just pick another case where the evidence is strong. They also have the responsibility to independently review the evidence from victims and police. These procedural checks didn't apply in the post office cases.
The procedural checks I mentioned above aren't fool-proof, but they're something.
