Translucency
MacOS Design Review
The trouble with translucency is that you can no longer check if your app “looks OK”, because your app has no appearance independent of what’s behind it. Instead, you have to check that your app looks OK when on top of light apps, dark apps, colourful apps, drab apps, and so forth. It never really ends.

Apple themselves haven’t even done this. Here’s how the Finder sidebar appears when the window behind Finder is light, and when it’s dark:

On the left, you can hardly see what’s selected, and on the right, the black on dark grey is murky and unpleasant. No professional designer would come up with either of these colour schemes. Yet, because of translucency, here they are.