I think there are two big possible differences in the blended family dynamic, which is why the advice people give varies so much and can even be contradictory.
The first big difference is the age of the children. If you marry someone with teenagers, you're going to have a completely different kind of step-parenting experience than if you marry someone with small children. It's hardly even the same thing. The younger the child, the more she will not be able to remember a time before you were around. You are going to be a huge part of creating who that child will be, even if only through your relationship with the child's parent. That's not true for teenagers.
The second big difference is whether the other parent is a normal person, with both virtues and flaws, with whom your spouse has normal disagreements and conflicts, or whether the other parent is batshit insane and/or interested in wreaking revenge in financial and other ways on their ex. A lot of stepparenting advice out there pretends that the second situation never exists outside your mind, which is really unhelpful when you're dealing with an actual nutjob. While it is true that under stress it can be tempting to bitch about a normal other parent who is pissing you off as if they are batshit insane, if you've ever run into a situation where someone's mental instability and desire for revenge are playing themselves out in family court, you know that there is a profound difference between the two.
So if someone's advice sounds wrong, it's probably coming from a diffrent place on the stepparent spectrum from where you are. If you've got stepchildren who still need help using the potty, it's just bizarre to hear "be a friend, not a parent!" Um, friends don't wipe friends' butts, usually... and advice to try to see it from the other person's perspective isn't helpful when the other person's perspective includes the belief that you are following her around and filming her (as my stepsons' mother likes to accuse me of).
I think these two differences are much more important than anything else - amount of visitation, number of children, etc,