It’s unreasonable to suggest that children having an issue with being step children is purely because of divorce
No one should treat a child badly.
But I'm not having for a second that all of this...
Different clothes
Different meal times
Different snack allowances
Different levels of freedom
Different expectation around phone/gaming use
Different town
Different friends/no friends because only there EOW
No dance/gymnastics/etc because away EOW
Christmas without father every year
Never just saying 'mum and dad'
Having step and half siblings
Feeling guilty for loving full siblings more
Never really relaxing at dad's because only there EOW
Is a result of having step parents and not just the result of having divorced parents.
I lived like that way before my mum ever met my now step father. In fact most children of divorced parents have to go through some degree of all of that whether or not their parents have other partners. None of the above would happen if the initial divorce hadn't. It's ridiculous to blame it on the fact you have a step parent.
No one should be horrible to a child, step parents included. As I say, they are not blameless if they are treating a child badly but it absolutely is the responsibility of the parent to get their child out of that situation if it's happening.
If my dad had allowed me to be treated badly by his wife, he would be the one I blamed. I was his child. She may be a horrible woman but he had the ultimate responsibility for me as my parent to make sure I was cared for and treated well.
Regardless though, the fact I had a step father is irrelevant to any of what the OP said, I still had to go between two houses, see one parent less than the other, have Christmas without one of them, not be able to say mum and dad etc... Because they were separated.