Comment by weinzierl
The elephant in the room is Han Unification but there are plenty of other issues. Here is one of my favourites from another thread just two days ago.
https://news.ycombinator.com/item?id=44971254
This is the rejected proposal.
https://www.unicode.org/L2/L2003/03215-n2593-umlaut-trema.pd...
If you read thread from above you will find more examples from other people.