An interesting thread discussion - thanks for starting.
I wanted to expand on the often said "you knew what you are getting into..." from a personal perspective.
When I met DP, very early on there was transparency on both counts about where we were on our journey in life. Me, 5 years older than him, been single for over a decade, with 2 adult children and leading a very active, independent life. DP did and still does, tell me that one of my qualities he admires and loves is how adventurous, active and outgoing I am. Him, just separated from a gf (rebound relationship after his marriage), 2 kids who live with mum and he sees EOW. When he was with exwife, he was ultimately the main caregiving parent (mum is not particularly hands-on nurturing) however his work/living situation meant kids stayed with mum.
Because he'd had a rebound relationship and it had troubled his kids, he had come to the decision with his ex-wife that he would wait 6 months before introducing a new partner. Very sensible and appropriate I felt. The first intro was in a neutral child-centered location. This all felt very measured, balanced and appropriate. We didn't move in together until we had been "dating" for 14 months, so that first+ year or so I only saw the children on occasional weekends (I would arrive on the Sunday, a few hours before they went home to mum so as not to intrude on their time with dad, but enough to start to build that relationship with them). DP and I conducted our relationship around his time with his kids, we would go off adventuring, travelling, exploring etc. On his weekends with the kids, from an outsider's perspective, it was heartwarming to see - a very involved, attentive, active dad. As it got close to moving in together, I began spending more time with them all. I had no reason to believe that I should have any concern, joining this family dynamic when we chose to move in together.
However, the realities of living together are completely different to any expectations I had. Little did I know how much the ex-wife would permeate the home. Either by proxy through the SC attitudes/behaviours or directly by her messaging intrusively (e.g. at the moment we are buying a house and ex-wife feels it appropriate to text DP to ask questions about the layout of the house we are buying). Ex-wife alienates DP, and this attitude of hers splurges out through the SC. They are contemptuous towards him (and his family). She uses the SC to relay messages that should take place between adults. She refuses to speak to him and only texts/emails. She is manipulative and consequently this is modelled to the SC who are in turn manipulative. The other thing I didn't perceive, was that DP would be (IS) a Disney Dad. He's full of guilt at the situation that the SC have been through, to the point that there are no boundaries and the SC are completely indulged. The kids are never 'disciplined' (I use that term loosely as I know it's a contentious issue, but there are no consequences or repercussions for bad behaviour). The kids are also very lazy, unmotivated and inactive. Hence, for me an active, adventurous, motivated person - I lose my DP and our relationship on those weekends.
Some would say that I am fortunate as I do not have to lift a finger for the SC - but equally, it has left me completely alienated and excluded. When the SC arrive it's like a "them + me" set up, I feel like a stranger and visitor in my own home.
So at this point, after rambling on to set the scene, I want to return to the OP, asking - why do SM get such a hard time??
Last night after another fraught evening with the SC. I was wondering, when will DP actually do anything to make an active change to improve the dynamic? DP "knew what he was getting into..." *eye roll. Where is his accountability in all this? When is he going to work on overcoming guilt and step up and parent these kids, that are ruling the roost? When is he going to start preparing these children for an adult world that will not indulge them? When is he going to put himself (and his kids out) to bring me into their family dynamic? When is he going to put in boundaries with exwife? Because I am on here (seeking help, support, unity), I listen to step-parent podcasts, I have a stack of books I've read, I've just started therapy, I talk to friends and my adult daughter about being a SM. I am doing what I can to be as best a SM as I can be but I feel like I am howling at the moon - I can't possibly be a "good" SM if I a never allowed the space or opportunity to do so.
However, that's what conditioning has led society to believe - that SM would be the issue here, fixing and making all the change (and perhaps that "I am the problem"). I'm just one cog in the wheel that makes this blended family turn.
Having read through this thread, seemingly DP/DH's don't take enough accountability and are not challenged enough on the impact their behaviour has on the loved ones around them.
(Sorry this turned out so long)!