PBR rankings in my state are not overly accurate I believe because they just started our state last year. So they are still trying to build a name here and they did not start scouting HS games till the season was half over.
I image it will get more accurate as the years roll on. I also would see them being more accurate then PG since they will watch players in other events that they don't run (ie HS games, non-pbr tournaments, etc...) where PG only looks at PG events and even then halfheartedly unless you are an identified national stud.