Based on what I listed in my last paragraph? I think you severely underestimate how different each 6* actually is, based on how varying their stats can be in addition to their other mechanics. Look at how useful Double Attack became because of extremely high attack stats on 6*s? It was definitely one of the worse Tough traits one could have on their weapon. Now if that was an extreme example, look at Stronger Attack then. Also one of the worse special weapon stats became useful with the release of Multi Attack ARs, that can utilize special weapon stats.
In regards to AD being stronger than impair before 6s, I strongly and vehemently disagree. Before 6s came out, impair was definitely more impactful than AD. There was no easy way to cure Impair unless you have a certain healing AR, and that could be blocked if you were Impaired, by any means. With how shield characters were in the 5* era, basic attacks was rarely fast enough to take down a shield character before the enemy could decimate your team. The 5* era itself, was largely about fast and devastating ARs. Scopely also released a 5* Tough shield in Garrett, so I don’t see why just because there was Shieldchonne, AD was extremely stronger. In fact, the only reason why you’re probably annoyed more with AD now is that you’re seeing it in combination with Shieldchonne more than usual because she recently became a F2P character with the ascendance upgrade. Ever heard of causation doesn’t mean correlation?
I’m not saying that a single trait being worse than the others doesn’t matter, but rather you can’t expect everything to be perfectly balanced in a game like this, much less in any game. There is always something that will be unbalanced due to the shifting meta. What about “bleeding when defending”, “ATK Down when defending”, “DEF Down when defending”? Many of those special stats have been relegated for one reason or another.
Since we’re also talking about seemingly trait specific weapons, in the past and current meta, hands down “Stun when attacking” is easily 10x much better than “AP Down when defending.” Shields aren’t a big deal right now, so why are we relegating ourselves to just discussion with the best special stat to use on shield characters? The point is, AP Down is clearly not the strongest defending special stat for Strong characters, but what Strong characters lack, they make up with one of the strongest offensive special stat in “Stun when attacking” as well as a unique special stat in Revive. Just like how Fast characters have “%AP” special stats, that were useful in specific situations, they also have AD.
And how has the meta changing helped characters like Gator and Tough Governor? They’re getting 6* versions… That’s going to be most useful they’ve been since their release, since they were largely not useful to begin with.