For years apple mail on iOS would always show 1 unread message in "all mail" for gmail.
On my iPhone 11 pro (and the 6S i had before it) the camera app will sometimes only show a black screen and a freaking reboot is needed to get it working again. I've missed out on many moments with my toddler because of that.
Multi-monitor support was so good on MacOS up until maybe 5 years ago. You could easily PIN apps to certain monitors and virtual desktops on specific monitors. If you started an app on a laptop and then connected to external monitors apps would automagically go where they were supposed to. Now it's a complete mess.
MacOS's UI seems more jerky in the last two major releases.
Time machine's performance has slowly gotten worse and worse, leaving me having to cancel backups just so I can go home.
I could go on. The worst part? These have all been ignored open issues that other people have submitted and for years they just sit there...open.