I just played Pantheon as support (autofill) because I didn't want to be a babysitter
I went 5-0 with couple of assists and I got an S+. Really, just that should tell you enough about this grading system. I've had games where I've been working my ass to play well, and never gotten an S when I thought I should. This is all in response to not getting a loot chest in days, with keys just being flung around like they even matter (I haven't dropped below 2 keys in months, always have shards to craft new ones).
P.S. This was in ranked, btw.