Well, everything's relative, so once you start asking for subjective ratings, any system will necessarily fall apart!
It's really just an opinion system, and you can read enough good or enough bad opinions to know whether to do more research or just walk away.
There are a ton of ratings for the M50/1.7 as well. It's a cheap lens, but good, and sharper than the 1.4 version at f/1.7 to above f/2 (or whatever). So why not have one? Who knows what else people who review the lenses have? In some cases they tell you, and they tell you why it's better than the rest, and they give it a 10 because of how it compares to whatever else they've used. But some might just buy it right after the kit lens and marvel at how it performs... A new standard, a 10!
You can really only compare to your own experience, so all ratings should be taken with a grain of salt, or just for another opinion.
Oddly enough, the M50/1.7 is my only M series lens, it was the first prime I bought and I used it almost exclusively on my K1000 when I first got into photography. I don't use the lens much any more, but I'm holding on to it.