If you hit the * button while in the browse screen (before you select a video), it will enable/disable the stats window during playback. You used to be able to toggle it during playback with the same button, but now you have to be outside playback to turn it on/off.
I don't have 4K access to Netflix, but when I did I had no issues using my Roku 4 and Ultra compared to the native Samsung app. Of course, there have been updates to both the Samsung and Roku apps since then, so things might have changed. I know that 4K YouTube looks better on my Ultra than the TV app, and I don't believe the TV app supports HDR, which the Ultra does.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.