OP..
I think that this topic is one where generalizations isn't applicable. It is on a case by case basis.
For every step father who was an a$$hole towards his step kids, the same amount could be said about step fathers who do right by their step kids.
It really depends on when the step father came into the kids lives, who the kid's mother is, her age and maturity level, their economic status, etc.
But, if generalization is brought into the equation, then perhaps it could be said that typically step parents treat step children differently than biological children. It may be rare that step parents treat all kids equally.