I don't think you do know what you're getting into when you meet a man with DCs. You don't know how his parenting and yours differ, you don't know what the DCs will be like, how clingy, how stroppy, how independent (or not) compared to your own DCs, how involved/reliable the ex will be or how your own ex will deal with the new situation.
Obviously we all go in with high hopes, DCs are excited to meet new GF and her DCs etc, dad is on his best behaviour and everyone is interested in doing things together.
It's like any kind of dating, once the initial facade wears off you're left halfway down a road with someone, already involved, DCs part of the package, realising that your parenting styles don't work together, that the parent who doesn't have full time access treats their DCs differently to the resident parent.
The RP who has all the donkey work not just the fun times and doesn't have guilt issues or 'let's make the most of our short time together' issues or 'I don't want to tell them off as I want them to enjoy their time with me' issues is seen as mean and stroppy, while the NRP is seen as being a 'good time' or Disney dad, all about indulging his DCs (& hopefully yours too) rather than being responsible and sensible.
Unless you've already been there before, imagining that this is how it will be seems like a very negative pessimistic attitude, so you go in with your rose tinted glasses on then get told that you knew what you were letting yourself in for!