I love the image quality of OLED displays, but I worry about pixel dimming (similar to burn-in).
Modern OLEDs can make it 5,000 hours without appreciable changes in brightness (at least based on some OLED TV testing). I'm not particularly concerned with Apple Watches, iPhones and iPads because they're just not "ON" enough, the always-on displays sit at low brightness (high current drives OLED dimming), and because even for things like the clock on the iPhone it would just take ages for the pixels in those changing digits to "dim" (though there may be some common pixels used in most/all digits that might start showing their age).
I'm more concerned about future OLED devices, such as display monitors which can realistically be on 8, 10, 12 hours a day while displaying static content like the menu bar, scroll bars, the dock, etc. Back in the 80's and 90's when CRT screen burn-in was a very significant problem, "screen savers" were invented to help prevent it -- but I still had terrible artifacts burned into the phosphors of my CRTs. But back then people didn't use their monitors as much as they do today (my 3 monitors today don't even have a chance of starting the screen saver unless I go to lunch). Hopefully they can improve the dimming effect before OLED gets to Apple monitors (or maybe they already have).