but is the assumption, then, that 'non-blended' families always "work"?
There'd be far fewer blended families if they did. Which I think is something that gets overlooked in terms of factoring in the potential pre-existing distress, pain, sadness, or even trauma of the children being blended.
Many children in a BF have already had to face an adjustment when their parents separated.
Followed by another adjustment when one of their parents created a BF with them in it. At a time when they may, or may not, have come through the other side of the original adjustment.
They may then face yet more adjustment if their other parent blends them into another family.
Possibly there will be another serving of adjustment if one, or both, of their parents add new children with their new partner to the existing blended family/families.
Given that second pairings tend to fail even more often than first pairings, they may have another "adjust on demand" phase still to come if one, or both, of their blended families collapses, separates and scatters. Which may, or may not, provoke a distinct sigh of relief for some the blendees, but all the same, will bring more change and emotional content to be coped with.
All romantic pairings carry risk for the all people in them. But blended families are more likely to carry an increased potential for successive "high stakes" adjustments being required. Particularly of small, powerless, choice-less people. Who may, or may not, have digested and discharged one large chunk of adjustment before the next lump is put on their plate.
The reality that not all non-blended families work is perhaps why blended families have such a tough time getting all their members to flourish within the unit.
One of the more common pre-requisites for a blended family to come into being is the collapse of previous relationships. That reality often goes hand in hand a significant, persistant degree of loss, hurt and grief for a good number of the involved parties. That emotional content will be part of the human foundation upon which a good number of blended families must build a home they hope will be happy.
It doesn't scream "reliable foundation for most of the people, most of the time" in any context. Let alone for a family that may have to endure more shockwaves, from more directions, than the non-blended variety.