No matter the ppi, UIs still look worse on an OLED compared to an LCD. Simply because they don't have pixels arranged in a grid, and UIs have lots of straight lines in them, those lines will always look fuzzy, even if just a little bit. High-density "retina" LCDs, on the other hand, render straight lines extremely crisply.
Isn’t that in part due to Apple being slow to adopt OLED along with its unusual subpixel arrangement which requires higher ppi to look good.