There are three major things that are realized immediately after updating One UI 5.1 that I do not like first is that the lock screen used to have the ability for users to simply tap on the user icon to switch to other users, but now it's turning to a super small icon that's around the same size as a status bar icons and you can't press it to change users anymore. The color palate theming is removed from the volume panel. Samsung Dex in One Ui 5.1 has a really weird quick setting which is that first there's not much button that you can actually tap on within the Samsung Dex's quick panel anyway which means it should easily have space for all the buttons to be in one page which is what all the past Samsung Dex did, however now for whatever reason it's two pages.
