I would say the main downside of C++ is that it carries around a lot of baggage from the last 40 years. Even if you decide to use a modern standard in your own code, you’ll have to deal with libraries that can’t use many of the new features because they need to stay backwards compatible with old code bases. Also, a lot of tutorials and examples still look like they are from pre-C++11 times.
I recently started a project that would have been a great fit for C++ (simple 2D game engine + editor) but the way Qt has to reinvent half the standard library, GTK basically asks you to write plain C and gtkmm had some other issues that I can’t even remember, all the fun of working on the project was gone. I decided to switch to C# (which I use for work anyway) and Avalonia and so far it’s going well. Not being able to use C++ still nags at me anyway. I would have loved to get more comfortable with modern C++ but I just couldn’t keep up the motivation.
I’ll try again for the next project.
Avalonia offers an unoffical Linux backend for MAUI and there are some other less-maintained backends. I have tried neither of them (I really should) but they might work.
The nice thing about WPF, MAUI, Avalonia and Uno is that they are all extremely similar in concept to the point where you can sometimes just take an existing UI, change some namespaces and class names and get mostly what you want. (WinForms on the other hand is very different from all of them)