My problem with this is since Zariel is so obsessed over karlach as to order minthara to track her down and bring her back to hell there isnt anything stopping her from just sending wave after wave to kill everyone but karlach and then take her all over again. Without anything more as far as content goes its honestly one of the most depressing endings.
I think you mean Mizora, not Minthara, but yeah, that's what I meant about "questions and fears". Personally I can freely be delusional (or maybe justified, who knows) in believing that my Tav (and perhaps friends later coming to help) can take Zariel down and all that, but it's also completely understandable that being told multiple times that a powerful archdevil will be coming for her relentlessly does not exactly instill confidence.
Wither's datamined lines do seem to imply that the default state of that ending is supposed to be "Karlach and whoever went with her are freaking decimating Zariel's forces and grow stronger for it", but they're not in the game, so. We'll see what actually makes it into the objective canon eventually.