I can already hear the howls of outrage.
It's a soft check, as is the one in Hells Lid. But you know what? The fact that it exists is itself a step up. No other expert in Stormblood has one at all. A tank and a healer can do Kugane Castle as a duo just fine, albeit slower.
Dungeon ones are also heavily influenced by what kind of group you get paired up with. A pure healer, a tank who never leaves tank stance, and a BLM who doesn't understand how to use Enochian are going to make the job of that other DPS player significantly more difficult, and those three likely won't even realize they're the ones doing something wrong because the game does nothing to surface that information.
So the difficulty with tuning this is that if you want the DPS floor to clear the place to be X (for DPS) and Y (for the tank), that means all three are at that level and the healer isn't factored in at all (as they've said). Get a savage purple parsing healer in that group and suddenly a DPS can be nowhere near X but still clear just fine. In another group they may fail it entirely, and that player has no idea why.
It also can't take comp into account because DF will give you all kinds of suboptimal comps. Those need to be able to clear it if the players play reasonably well. So a optimal group will destroy it and probably wonder how it's possible to fail.
So, I mean, I think having a DPS check like they added in dungeons is a good thing. Maybe the next ones are tuned a bit tighter, but it's a step in the right direction. By itself it can't exactly solve the problem.