SUMMARY Right now you document that qtmultimedia is broken and should not be used. However your cmake file says that qtmultimedia is compulsory and kasts can't be compiled without it. Which means that users will have the option in the UI to use a broken backend that is known to not work. It should be made optional, so that it can be removed from the dependencies, so that users will not see that option in their GUI and will not have the possibility of picking it.
Well, the situation is complicated, and the current approach is intended as a compromise. Firstly, while qtmultimedia has several known issues with no known solutions, it is not completely broken. It does provide the basic functionality (although buggy). In very rare cases it can even play or stream files that VLC cannot. Secondly, Kasts aims to support linux, freebsd, windows and android. The only common backend that works on all these platforms is qtmultimedia, hence it being mandatory despite the known bugs. On linux, yes, by all means I would like to remove qtmultimedia and gstreamer and make VLC mandatory. However, this will then interfere with distributions like fedora that are very hardcore concerning codecs and don't even ship VLC in their standard repositories. Let me think about better solutions that won't introduce more problems downstream than the current one.
Perhaps it could be easier to patch it out in debian instead?