I have found the FeathersUI focus on Openfl integration certainly helps make it feel more streamlined, it works the same way as the rest of Openfl and when your working on a project and you only use a few UI elements to speed up development in certain areas, not having to think in a different way is much easier.
Certainly HaxeUI is more abstracted to enable flexibility and potential speed, and for some developers that would be a boon, but if your focus is solely Openfl, FeathersUi slots into the workflow more easily.
Of course HaxeUi has native integration with Openfl, with the ability to style within code directly, but that XML backend is still there, so when you output your html code for instance it automatically creates XML files that need to be included for it to work properly. I just find it more detached than FeathersUI as an irregular user, and that slows me down.
Of course both provide an amazing resource that we should the grateful for 😃