I have not attended any tournaments which allow Khorne teams. (I consider a "tournament" any environment which lets you start with a fixed team and skills, as opposed to a league, which starts with rookie teams that develop with SPP). I don't think they would be very good in a tournament. It's not only that they need a lot of skills - lots of teams do. But in my opinion, they need some well developed key players, like a Blodge Bloodletter or two, a killer Herald, and so on - most tournaments either just disallow skill stacking on players or make it prohibitive. Khorne position players have quite decent skill access, which is good because initially they're somewhat expensive.
They are certainly not overpowered in any environment. Somewhat the opposite actually, a lot of the criticism boils down to fluff "Any Khorne team should just pitch clear any opponent by turn 4". I haven't heard anyone who's actually played against them complaining about game balance issues, and mentally I admit that I tend to dismiss criticism of the team as old farts complaining about the game changing, or cyanide haters complaining about anything that cyanide does. Apologies to anybody who has a legitimate gripe with Daemons of Khorne.
I have nothing to add about Brettonians, never having played them or against them

Edit: I typed this up before looking in detail at your attached graphic. "Group Two" and "Group Three" seem to collectively contain teams that are often considered simply "Tier Two". So I say Khorne is Tier Two, and I can't really buttonhole them into Group Two or Three. Depends on the environment. If they can take a lot of skills, they'd be closer to Group Two. If it's more like rookie teams, they're more Group Threeish.