I don’t think it’s that straightforward or simple.
Yes, sometimes it is a DP issue. From the boundaries they set with the ex. To the ways in which they prioritise (or don’t) their existing partners. Maybe difficult issues that they refuse to confront and prefer the head in the sand approach. There are many issues that can be DP issues (which can also be reversed to us, the other partner in the relationship)
However, I completely agree with bananas in that there are certain things that cannot be controlled. And that is both the ex, the impact / influence the ex has on the DC and the DC themselves and their emotional journeys which they need support with as they traverse this world they never wanted to be part of.
There is then also the added layer of a DP who comes into the equation as a SP. Depending on their values and their expectations, they too inevitably add a layer of complexity to the mix.
Second families come with a deeply complex mix of issues. They require compromise, communication, honesty and an ability to listen. To your partner and the children. Similar values are paramount and if you’re even considering living with someone, parenting and discipline must be discussed. You cannot have a two tier system for resident children and non resident children.
If your values do not align, if your communication is not strong and robust, if you cannot honestly voice your fears and opinions and be listened to, respectfully. If you cannot compromise. Then you should not even think about making a living situation more permanent where everyone is forced to co-exist like some sort of pressure cooker environment waiting to explode.
So, no I don’t think everything is a DP issue. Exes hold a lot of power, and they either use that power for good (to reduce conflict) or not. And that is something no one can control.