Hi folks, thanks for all the answers. Yes, I know, mixing both might be causing troubles. But sometimes I got no choice.
For example, the NVidie kernel driver can not be compiled on newer kernels than 5.10.22. Reason is, that in the herader files of the newer kernel package some files are missing, which the compiler needs. I filed a bugreport, but the devolper will not fix it, as they claim it an NVidia-issue. That does of course not explain, why it on one kernel versions builds and the next version not. However, I do not want to mourne about it, just tell, what I mean. Then I saw, in testing the famous application "digikam" will be removed. So, if one did use digikam for a long time, he will be surprised and maybe angry, that suddenly digikam has gone. These are only two examples, which can be trouble for users. You can of course say: "Hard luck!", but the only way to avoid this, is to mix stable and testing. Or (in case of graphics driver), buy a new hardware. As we are only one month away from releasing the actual testing (which is in freeze state) to stable, I could upgrade now, if I want to. I tried a dry-run, and saw i.e. digikam to get being uninstalled and some others. Also it tries to uninstall my NVidia package, which is worse, as it is running and I have to avoid this, because it will break my system. As it is a notebook I cannot change the graphics card. And no, nouveau is NOT an option! Worse: I never will get it back, as once deinstalled, I will no more be able to build it again, as also the needed compiler and header files are gone in testing. Yeah, hard luck, ey? As I said, no mourning, just telling, why I believe, deleting packages from the repo is not the best idea. However, this is only my personally view and maybe I am the only person in the world, who might be affected. Anyway, I understood all the dangers, and as I (believe) I know, what I am doing, I hope upgrade to the next release will be without major problems. Thunbs pressing!! Best regards Hans