After One UI 3.0 update I noticed few issues which affects usability and functionality of my S10+:
1. Edge Lighting now missing an option to choose color I want for the app I want. It was very useful to see different colors for different apps. I had it's own color for every app I need. Now it's only possible to see app color related to app icon.
2. Animation speed became too slow. It can be solved in Developer Options by changing Animator duration scale from 1x to 0.5x but in this case Recent Apps animation will be not smooth, at the moment when it opens you will see laggy effect.
3. The location of the Power Off and Settings icons in the Quick Panel are lifted all the way up which defies the purpose of one hand usage. I'm left-handed person so I have to use both hands now to open settings from the Quick Panel.
4. I don't know why the Lock Screen clock style now missing cubic style clock. It was my favorite.
5. The Lock Screen now have that ugly LOCK icon. It's too big and useless. It's absolutely unwanted thing on the Lock Screen. How can I hide it? It strikes the eye and ruin the Lock Screen aesthetics.
I hope this information will be useful.
Another one issue I noticed with the Navigation Bar. I'm using Swipe gestures from bottom so now when some app is running on the screen and I do swiping up to see the Home screen or trying to open Recent Apps I always accidentally touch that app buttons which are located close to the bottom of the screen. Swipe to go back have no such issue, only Home Screen and Recent Apps gestures. Swipe gestures from sides and bottom has the same issue and it also causes false actions.