Not sure that the cadence actually gets a special goal, I suspect my understanding shown in the video is simply wrong and they just have to kill all of my heroes to win. Mostly because I don't really get any information about that from the Chalice itself.
They certainly are a welcome change by now though. They include more enemies but also really different stakes, instead of gambling on realm status, you effectively are wagering the entire building and line, I am pretty sure you lose the building and everything in it. A weaker version of losing the entire domain to corruption. Finally I am pretty sure that the keep defenses all take place on different maps than the standard map pool. Overall it is a nice change once in a while.
The other thing these missions do is show that while the game is built around 5 hero squads, it does a fairly good job of adapting to larger squad sizes. It only really struggles in the Results screen where the UI expectations are most visible.
You can see that the extra heroes pushes over the end of the page, meaning we suddenly require scrolling. I do appreciate that they have put some thought into this situation. It can't be an accident that the sixth hero is partially visible which tells the user that hey, there is more information available, you just have to scroll down to see it. I think it works well as a solution to a relatively infrequent but expected event in this game.