I saw that logic has separation already into 6 stems. A must have in Cubase 16 as well, if you would ask me!
If you really MUST have it now there are solutions to help you right now.. like Spectralayers pro. Or Logic of course 
You don’t say.
True, Logic has a built in stem separation that is more advanced (and faster) than the 4-stem separation in C15 Pro. The free Spectralayers Go is on another level but offers 2-stems only and runs as ARA extension. So yeah, it would be nice to have an integrated stem separation that is more advanced and as fast as Logic’s tool.
Personally, I wouldn’t mind to have better stem separation but I’m also good with the one we have. I only use it to have a quick listen at drums or bass stems. Since I don’t actually use the stems I am not bothered about artefacts.
My guess is that we’ll see some new features in that department in the upcoming C16 version. Let’s see what C16 brings.