The only way to prevent any sort of rating scheme from being subjective is to have actual data such as Pitch/Roll/Yaw rates, overall speed, stall spin characteristics, ect. What a monumental task that would be! Otherwise we get to watch this subjective, and most likely mis informed thread pop to the top every single day.
It's foam folks.....if it's to difficult to fly you're out a sheet of foam, it's not like reccomending a scale built up airframe worth 1000's of $ to a new R/C pilot.
If you have to ask yourself if a design is too difficult for you to fly, then it probably is.
If you think it is....Go for it anyway, sooner or later you'll get it. My opinion is that a rating scheme might keep a potential builder from improving their skills by pushing them to the limit.
Never let anyone hold you back.