I'm about the same way. I also think one UI has come a long ways, but just not there with Nova prime. It's the small things that keep me with Nova. I always find something I'm missing from Nova when I try one UI. Nova has been using gestures long before Google and Samsung has, that's why I don't get excited with Google's gestures, I'm like I've been doing that for years. I've also been doing double tap to turn screen of and lock phone. Nova has a ton of gestures you can do.
Other reason I keep Nova prime, I know how it works. No matter what phone I get, I know the launcher. I can get a Motorola, LG, whatever phone and I know how to work it. I have been with Samsung for some time, but even with Samsung I don't have to learn the changes.