Dark mode is great when you're using a screen casually close to bedtime and don't want a painful bright light.
It's also great when you're doing image or video editing and need to be able to see full contrast in the dark areas, without being overwhemlmed by bright light surrounding.
But it's objectively far worse if you're reading significant amounts of text, for the simple optical reason that light bleeds inside of lenses, including in your eyes -- that white "pollutes" black but not vice-versa.
Thus with black-on-white text, letterforms stay separate, clear, and legible. Any light spreading simply makes the letterforms appear, say, at 10% brightness instead of 0% brightness. Zero problems with legibility, still plenty of contrast.
But with white-on-black, letterforms glow with blurry edges and connect into each other. Not on the screen, but on your retina. The same way streetlamps at night appear to have a halo. Words take more effort to read. Now granted, if you're 15 years old with perfect vision, it may not be particularly noticeable or objectionable. But the older you get or worse eyesight you have, the more it becomes a big problem.
So dark mode is nice to have, but for most apps going dark-only is objectively bad for accessibility, and legibility generally.