I think quality of play should take the most precedence in reviews. I'll compare two different 9s to illustrate:
Mill Park is a new 9 near Salt Lake. The signage is beautiful and incredibly durable, tees are from the sidewalk or are cement, baskets Mach X, grass that you could enjoy barefoot, seating and trashcans available everywhere. But the throwing is tepid and repetitive, and the safety hazards are legion. If reviewing it, I'd probably rate it a 2.
Meanwhile, Takena Landing in Albany OR is a 9 with no signs, benches or trashcans, tees are carpet, its never mowed(tho the woods have been tamed enough not to lose discs), and it uses old Discatchers with 8-10 inner chains. But the obstacles, elevation and complexity of throwing are excellent, and it greatly resembles the much-liked Bryant Park across the river. I'd rate it at least a 3.
For the reviews I have written, I've got 9s rated to 3.5, and no 18 higher than 4.5, so I think 9s/18s compare well in relative terms. I can't say I'd never rate a 9er as a 5, but it would have to be beyond compare to top my highest-rated 18.