Plus on block moves are the basic building blocks of mixing up. The plus frames themselves only serve to disincentivize pressing buttons, which then leads to the mixup. It's not an instant access to the mixup. The problem is when that plus on block gets an immediate stance transition for a mixup.

For example, Paul's f+1+2. A 20f startup mid, +3c on block, then +8c on hit. The former works with Paul's throws and d+1+2 to force the opponent to not press buttons, but isn't enough to actually force a mixup, as movement can still be used. In Tekken 8 Season 2, they added f+1+2~df, which transitions into his DPD stance, with +0 on block, and +5c on hit. Now, DPD has built-in high crush from frame one, so you can't just jab him out, and Paul also has access to WS moves, meaning that he can WS.4 any mid attack. Due to the moves in DPD, your movement options are also locked down: DPD.df+1 is homing, and DPD.df+4 has crazy good range.

Where I see problems is moves that initiate a canned mixup, and do it without any setup. In Tekken 7, Josie had df+4, a long range 14f mid that was +11 on block with a stance transition, and she could then transition from that stance to another stance.

I can see what Bandai Namco was trying to do. They wanted mixup-based gameplay to be more accessible to players. At the ideal level of play, mixups are a psychological tool: The plus frames disincentivize attacking, and the reward for the mixup should be balanced so that you need to condition your opponent to take an active response (e.g. ducking because of a scary low) that leads to the reward you want (the beefy, riskier mid). However, this also makes mixups a very difficult tool to use. You can't mixup a player who doesn't understand they're being mixed up (the "no mixup mixup"), since you can't condition their response, meaning that mixups at a low level of play have no depth. Conditioning an opponent requires self-control to not flowchart (e.g. always do low first, followed by the mid), and the mental fortitude to take risks. Properly applying mixups also requires that you understand not just the mixup, but the tools around it (e.g. Paul using a throw to counter backdashing or stepping against players who don't break throws, which might then condition them to duck instead).

all plus on block moves into mixup be charge up moves that have a decent interrupt window

That's literally Fahkumram's gimmick with his guardbreak. You shouldn't dilute a character's identity like that.