My experience when I split from my ex was that we both had to work at making the transition of having time away from me and the bed/home DD had known since birth, as seemless as possible. That involved me going to my ex's place, and spending time, regularly, with him and her while she got used to his new place, and away from me, as I was her main carer before the split. It took several visits, at times 3 or 4 hours in length, before DD felt comfortable for me to leave her, with her dad, and then progress to staying overnight. I had no issues at all with the level of care he would provide her. It all came down to what our DD was comfortable with, and how long she took to being settled with a new place to spend time in/sleep overnight.
I think if you don't have the sort of relationship where that kind of build up is possible, then I can see how the whole situation of a protracted, slow build up, can happen and no doubt feel frustrating to the NRP. As harsh as it is for that to happen, for me it's all about the child, and how they cope with that transition. Dismissing the child's anxieties, insisting that they'll adapt, ignores how big a step it is just to get used to a new home/new set up follwing on from a split. My DD was always quite clingy with me, had strong separation anxiety, and often said she didn't want to see her dad/stay with him/stay over night. But, it was nothing to do with not wanting to see her dad, but was more to do with her anxiety at leaving me. She loves her dad so it was never about him not caring for her/treating her badly/neglecting her. I had to do a lot to help ease her anxieties, and luckily me and my ex were quite amicable so he understood the need for me to be there, for her sake, to help ease her into the new set up - even though he could have taken offence at me being there, taking it as a slight on his ability to parent our DD. He never had an issue with it, and DD eventually got very settled over about 3 or 4 months after ex got his new flat.