So I finally got the stable version of One UI 4 for my Samsung Galaxy s21 today.
I've never experienced Android 12 before as I never got the beta.
Now considering this is the stable version I've found the material u buggy. For a start once you choose your icons to change colour to match your wallpaper you can NEVER get them to revert back to the original colour. Even when you change wallpaper again and turn off the feature to change icon colours you'll just be stuck with the colour you selected previous.
Also any apps in your app drawer which are in folders will always be one colour behind, meaning it'll be the colour scheme of your previous wallpaper and not yout current one. These may be only small issues but it ruins the whole material u concept.
TBH both have their Pros and Cons but I love my S21 Ultra overall . Just cause Samsung has a wider ecosystem, like the smart tag and stuff. And their option to connect with other smarthings. Also they work closely with Google which helps.