I don't have unpopular UI opinions, but I do have opinions that I don't see people echo much, yet.
One of the worst things about UI in 2025 is that almost everything most people use on a computer relies on it, more than ever, and yet it's also at its worst point since the days before mouse driven interfaces. Companies used to be much stricter about their interfaces, how they worked and looked. Now there are tons of bespoke interfaces where everyone decides for themselves how they work, and assumptions made by one program work the opposite way in a different one.
Switches have become way to obvious to what "on" and "off" is. Even when they state something like an option is enabled or not in text, it often isn't clear whether it's saying this is what the state is now, or this is what it will be when clicked.
Icons have become way too vague and arbitrary as to what they mean. The Hamburger menu was bad enough, but some of the icons have gotten way too abstract. At least the floppy disk for saving was a convention.
Web pages likewise could use a lot more consistency and visibility. The new Digg, for instance, hides its user block function behind a light-gray three-dots button on a white background. The only options on that menu are to Report or Block that user! Why is it three dots, and why is it so hard to see?
Microsoft's "Ribbon" interface remains a terrible idea. At least with menu bars you know all the functions are there, somewhere, all represented by text. With the Ribbon, everything's a toolbar button, and with many of them being different sizes it's harder to scan through them to find the option you're looking for.
As someone who doesn't generally play with children's toys, i still say the answer is never.
As Randall Munroe said way back in XKCD #150, we're the grownups now, and we get to decide what that means. Life is too short not to spend as much of it as you can doing what you want. So long as it doesn't interfere with others, go wild.