So let me explain how I think it works, top to bottom.
Ardenweald - a kind of Limbo where those who’ve done terrible things, albeit under duress, are given time to heal and come to terms with what they’ve done; that time can eternal so being kept in a slumber for that long may also be some degree of punishment.
Bastion - Mind wipes and an eternity of servitude carrying other souls to the afterlife. Doesn’t seem like much fun to me. I also got a despotic grand-show of architecture vibe from the zone, maintained by slaves, but obviously not for slaves.
Maldraxxus - a strange indifference to the moral compass of its inhabitants, but mostly not good and just spoiling for a fight and dominion. A standing army kept busy fighting to defend from what?
Revendreth - the last chance saloon, a place for atonement for one’s sins.
The Maw - Eternal damnation.
Is that a good or bad thing? I don’t know, but it probably explains how some people are finding it hard to relate to some things. I found the whole place up to Revendreth utterly hateful and couldn’t give a damn about killing dead people and sending them presumably to the Maw. But I’m glad I pushed through and tried to understand it, and got to Revendreth where people were much more relatable and ultimately wanted to be better.
As for Sylvannas, it seems like she wants to be Kerrigan now and kill whatever deities put this system in place. She knows probably better than most that eternity is a long time.
But it begs the question, is Shadowlands all the Bad Place? Are there are other places that make her wrong? What happens to those redeemed in Revendreth or healed in Ardenweald?