sneak attack and banish etc don't work with the new system, you only roll 'weapon die' again, (and probably only once if its like before) basically they are trying to reduce damage swings a lot more.
new system crit = +d12 max. old system crit for some builds = +12d6+d8+d4
game design wise it makes a lot of sense, reduces crit fishing, monster fights are less likely to be one shot by luck, less variation in dmg from build to build
but the problem is, psychologically, some players live for that crazy synergy, and the big feeling on crits. they might be giving that up for more stable encounters/battles
as far as 'nerfing' mages, not really. much more consistent access to forcing disadvantage on saving throws increases mage damage. (every 'crit') gives inspiration now.
anyhow the crit stuff is a test and will probably be iterated, maybe they'll buff crits in a different way or reverse it, but I'm of the opinion the changes are probably fairly well balanced, more than the game was previously. However game design isn't all about balance. its also about fun, the question is, will this stuff make the game more fun? I dunno
Where do you see in the document that sneak attack and banish etc. Don't work with the new system? Is it because it says Damage Dice are rolled a second time?
Either way, that's SERIOUSLY lame, and it still means wizards get the raw end of the deal. Barbarian hits with greataxe and Crits. 4d6 damage plus strength bonus. Mage hits by rolling nat 20 with Firebolt. Normal damage is 2d10. Nat 20 doesn't increase his damage. It's still 2d10.
