From my experience of many decades as a secondary school tutor, I would say most of the time when children talk about step parents, there is tension and misery ( from the children's point of view, not necessarily the parents)
I am going to say 75% of situations are unhappy, by which I mean most of the children are somewhat unhappy, or one or more child is very unhappy, or the situation breaks down because of parent or child unhappiness. Breakdown could mean the relationship between parents breaks down, or the relationship between a child and step parent breaks down to the extent a teen becomes homeless, or moves out
So to answer this, you probably need to know at least 4 blended family situations reasonably well, yours as a child, parent, or other people's.
I am expecting that some parents will vote that it is happy, when that isn't accurate, as I am aware this is quite common, and the child has a very different feeling than the parent.
However, I will be pleased if I cam completely wrong about this, and lots mare happy! I just dont see it in teaching.
YABU - less than 75% are unhappy
YANBU - 75% or more are unhappy