That actually makes stun punishers worse (back to being the trash they normally are outside of UE); they're terrible because of how relatively late into a fight does stun mode usually show up. A short mode gauge means that the opportunity for a stun punisher to be remotely relevant comes up earlier and more often.
If normal mode lasts 10-12 million HP, then raging mode itself won't show up until the 30's (exactly lv 30 if it's a 10 million normal mode bar), without using something like Acala's normal gauge pusher against wind. Assuming rage itselfs lasts for as long as normal, that's another 30+ levels on top of that before stun shows up for the first time, without using any % rage gauge shavers (although those would now be worth potentially millions of HP now if you want to get out of rage asap). And our experience has almost always been that rage lasts noticably longer than normal mode.

Thing is, there's not a lot of depth to Nutaku unions. For ult, a high of ~104-108 gets you to rank in the 90's, ~115-118 gets you to rank in the 80's. For unions in that range, you'll probably only see stun mode show up once, pretty late into the fight, and only if your union's pushing. Otherwise, on non-push days, you probably won't see stun mode.

For expert... how long does the normal mode last? Cause the wiki lists 6 million for raging, but doesn't say for normal. Conversely, I see lv 30 as the minimum for ult to see rage, implying a 10 million HP normal mode, but no numbers for raging.
Anyway, 6 million HP for raging means that a single rage bar lasts 40 levels on its own (as expert's HP is 500k base + 150k per level). So the duration before each instance of stun mode is (however many levels worth of HP normal mode is) + ~40 levels worth of HP for raging. Stun mode would just show up far less frequently compared to the seven sins' "all the damn time".

For rage punishing stuff? *shrug* For them, the times where they're applicable get smushed together from small frequent slices into big chunks like in most non-UE content. And they're still relevant outside of UE, so they're not worried.