I think there's a fine line that needs to be balanced in all step parenting situations (but majority step mother situations).
Which can be summed up as: is the step mother (or parent) allowed to set their own terms and boundaries?
If you're allowed to develop your step parenting role on your own terms - decide what you're comfortable doing, what you're not comfortable doing - and that is unequivocally accepted by your partner, then I think it can work harmoniously.
But time and time again on here I see situations where the step mother has unreasonable expectations and obligations placed on her not only by her partner but also by her partner's ex. And sometimes by extension her partner's and the ex's families.
They are treated like a nanny and housekeeper by their partner and his ex - who both end up benefitting from her unpaid labour, which is to her own detriment.
It happened to me, when I was on maternity leave and when I went back to work part time to manage the care of my own DC. DH and his ex jumped on the opportunity to hand DSC over to me since 'I was at home anyway'. This kept me stuck at home for many years. Unable to properly progress my career while DH and his ex continues to develop theirs. If I challenged the status quo I was accused of not wanting DSC at our house. It was classic 'contact by proxy', where DH considered it sufficient that the DSC was present in our household even if he was working late and I was the one doing the looking after.
My situation ended up causing a lot of resentment and my mental health really suffered - especially post-natally. I struggle to forgive DH and his ex for that even now. It's been hard. And if I'm honest, it has affected my bond with DSC.
But all of it could have been avoided if they'd just never taken the piss in the first place. Ironically, I used to really enjoy taking care of DSC and going the extra mile for them until I realised that it wasn't appreciated by either parent, it was expected and actually, the bar kept getting raised incrementally higher to the point where what I was prepared to do for DSC actually wasn't good enough in their eyes and they demanded more and more. If I resisted, I was the evil step mother.
Anyway, that was a very long way of saying that the thing that always ruins step dynamics is when the step parent is treated like staff by the 'first couple', so that they can reap the benefits to the detriment of the step parent.
If the step parent's boundaries are heard and respected, it can work really well.