I worried about the Nav bar...
...On my phone when I got it years ago.
Not because of burn-in, I never knew it happened on phones.
I worried in case the bar didn't work or disappeared and wouldn't return.
Now, I love it; I can disappear it if I want more screen real estate (or set some apps to do it) and it never fails to appear or operate.
The Nav bar can be made to match the UI theme, although I prefer it natural.
I have also set it to allow double tap off the buttons, which switches the display off, no button press required.
No burn-in visible, I looked, hard.
It is an OLED and it is hi-res, even by today's standards I think (563ppi).
It is less bright though than these fancy new ones though, although anything other than bright sunlight is fine and it kicks up the saturation/brightness violently when it is sunny, which changes the colours but leaves it more readable at least.
Having no UI buttons really need not be an issue and should be a benefit - it just requires them to work properly.