I've been a "SM" twice (both very long term relationships, but never married) and there was a marked difference between the two relationships.
The first, the kids mum was beyond a bellend. Spiteful, point scoring, probably the nastiest person I've ever encountered. But. My DP danced to her tune because "she'll stop me seeing the kids." She was 90% of the reason I left (and others after me), which she was thrilled about...if she couldn't have him, she was going to make sure no one else could stand to be around her constant abuse. The kids were frankly horrible as well, which was not their fault, they were a product of their environment.
The second, they both hated each other, no doubt. But were excellent parents. Never bad mouthed each other. Put the kids first, always. It was apparently a bit fractious before I was on the scene, so he took her to court, things were set in stone, and they both went by that. Our relationship was great. Together over ten years, and still friends now, just drifted. The kids were great, lovely young people, who have become lovely adults, and still keep in touch now.
Being a step parent isn't hard, unless one/both of the actual parents are idiots. In my first scenario, it was her...but my partner allowed it. All this "oh I have too, she'll stop me from seeing them" crap all the time. But never could be arsed to get a court order drawn up to remove the issue. Pretending he had to meet her ridiculous demands, when all he needed to do was deal with it properly. So actually? Both are the problem.
In the latter scenario, I have a feeling it started in a similar manner, but he nipped it in the bud and got it done, clear cut, through court. She then saw that any dicking about on her part wasn't going to have the results she desired, and so just got on with being a co parent, and actually, by the time I turned up, the pair of them were two of the best co parents I've met. Their lovely children are testament to that.
Being a step parent isn't hard. It can actually be great. Being a step parent with an idiot actual parent is impossible.