onX ratings are also pretty subjective. There is one user in my area who is rating a bunch of county-maintained gravel roads as 4s and 5s. People who live on the roads drive sedans down them! Even in CO I've seen some trails that were pretty under-rated, and others that were probably over-rated.
FWIW, I've found onX to be pretty useful when wheeling in areas I don't know well. It's great for CO for sure. When in WY recently, it seems that there aren't very many users. I found a dandy of a 1-mile trail that wasn't mapped at all. Around here, I know the area super well and don't really use or need onX.