I wouldn't say it sucks, just that it's a more complex, lower-level API layer than DX11 and earlier. Which is good in that it allows for more control over game performance, but the big downside is that it's more difficult and easier to screw up.

There were a lot of fundamental changes under the hood going from DX11 to DX12 concerning fundamental programming paradigms in the API that a lot of devs are still grappling with. It's probably just something that will take some time for people to get used to.

From what I get and I only have a very basic understanding of Graphics APIs is that DX12 puts much more respobsibility on the developer which raises the performance peak one can achieve but it also lowers the floor of performance when you don't properly optimize code.

On Windows you may be right. A buddy I game with regularly has had trouble with DX12 games crashing randomly.
On Linux they run just fine and frequently perform better than DX11 on Linux or DX12 on Windows.