Champion Mastery is one of the systems I use a ton. I have 120 champs at lvl 7, and am working my way through the rest.
The Good:
Having more to pursue beyond rank 7 is interesting
Having the first milestone be more chests is always nice
I like the reward of "Your champs Title" be tied to performance as an unlock.
The Bad:
The time gating of both rewards and achievements seems awful. Mastery has always been permanent. Having them be tied to Split for the reward of a title is really annoying, and feels like a FOMO approach.
Mastery Sets, from description, reads like it'll be another monetization metric ala Eternals (which goes against how the mastery changes were worded in the announcement late last year). I like Mastery stuff because its free and accomplishment based. Having once a split reward tracks that get pushed seems dubious.
Marks of mastery seem very easy to earn compared to S ranks. If I want to hit 7 on a champ already at lvl 4, then I would simply need 1 S grade. Hitting lvl 10 would require 2. If you could get lvl 6 before, you now qualify for lvl 10 mastery.
The system seems like an earnest attempt to improve mastery, but also a lot of it feels intentionally grindy.
The reward of title is a great idea, but gating it behind splits and requiring you grind out every 3 months is bad.
The idea of recommending champs based on mastery is interesting, but feels like a push for grinding.
The way marks are being shifted from "get 5 S ranks" to "Play 5 games, get 2 S grades ever" feels like a very low barrier of entry.