I agree with all of this. It's impossible to say what would have been better as an adult looking back. It's a risk benefit analysis and you've no way really of knowing. If there's abuse it's clear but when it's something like "unhappiness" well it can be pretty vague at times, and then it's hard. Unhappiness doesn't tend to be in every aspect of living. Although in my own case it eventually did permeate every part of our relationship.
If you have shitty parents or a shitty parent when they stay together then it's likely they will be exactly the same when split so either way you would be brought up with shitty parenting. That's the negative effect and it doesn't magically disappear when you split. Maybe if it's largely one shitty parent, by staying together the good parents protects the children somewhat?
I've recently split with my dh and I'm watching the impact and fall out. For me, it's the best thing ever, I'm good financially and the arguing has just left the building literally and that peace is lovely for everyone. But my children love both of us very much and rely on us both equally and turn to each of us for different things. We've removed one of those very steady reliable loving anchors for at least half the week. They are finding that hard. Actually heartbreaking to watch my youngest struggle with it and I'm not sure this has been the best decision for her.
And I've kept the family home and he's just moved 10 minutes up the road and they know they can pop in at any time to either house because they are both their homes.
I think it's probably better that we are separate because we are good parents just really awful partners...bring out the worst parts of each other. And I certainly won't be introducing any step parents... Who knows what his future will be.
Who knows what damage we've done to them and whether the damage occurred during the together years or the divorced ones.
Life is hard and you can never be sure your decisions are the right ones.
Sorry for the huge post, but writing that down has been helpful for me.