It's not such a bizarre assumption at all. People who post on here attacking stepparents and everything they do sometimes do turn out to be bitter stepchildren by their own admission.
I really don't like the word bitter in this context, it implies that you are choosing to hold on to resentment of your upbringing .. when really it is quite difficult to get past not being wanted as a child and made to feel like you were an annoyance, never good enough, or barely tolerated, unless you've been in that situation as a child with no autonomy, I'd suggest you refrain from flippantly labelling something you obviously have no clue about.
I wouldn't describe myself as a bitter stepchild, but I am a damaged one, and I will always point out awful behaviour in any step parents I come across, it is nothing to do with attacking them for no reason, and everything to do with recognising the attitudes and behaviours that I grew up with and wanting to make them aware how damaging it can be to inflict that on a child.
Caring step parents, and decent human beings, just don't use this language about children in their care I'm afraid:
She has no resilience and craves attention.
I'm at the point that I don't want to invite her anywhere as she always has to cause issues.
Told DH he is massively causing issues
Then after the crying, her and DH went off to do rides that we easily all could have done together.
It was DS birthday and it was just a shame that she needs the attention on her
I want her to have nice days out too as she doesn't do them often (including with her Mum) but if it's going to cause arguments all the time then it's not worth it.
She cries because it works with her dad. Exactly this!!
She 100% would not be talking about her own child in these scathing terms, so it is obvious to most on here that all OP sees is an annoying child manipulating her dad to get her own way, with absolutely zero understanding for why she may get upset when she is with them, OP doesn't approve of them spending one-on-one time together (also a massive red flag, and I suspect the root cause of the issue), and she has zero compassion or empathy for the child she has chosen to bring into her life.