I can only speak for Flatpak, but I found its packaging workflow and restricted runtime terrible to work with. Lots of undocumented/hard to find behaviour and very painful to integrate with existing package managers (e.g. vcpkg).
I also think Qt/QML is a very underrated technology. I have been developing a handwritten notes for Linux/Windows using Qt Quick for quite some time [0]. The experience has been a mixed bag though.
I've encountered tons of bugs (many of them still unfixed) that I had to find very ugly workarounds for.
Also, while a declarative style UI language can have a lot of benefits, it does also have a lot of limitations. For example, in my application I required a (infinite) canvas to rendering the ink strokes, which would be a perfect job for QGraphicsView, but there is no equivalent in Qt Quick. So I had to roll out my own component (which uses Skia under the hood), but that was quite painful. Since Qt 6, the Qt Quick scenegraph is rendered with a custom RHI backend (abstracting over Vulkan, Metal, OpenGL and DirectX) which I had a lot of trouble integrating with third party engines (I really wish they had a WebGPU backend).
QML is a great language to make GUIs. A few years ago I tried XAML, and it honestly kind of sucked in comparison (the verbosity alone made it painful to work with). I haven't tried Slint UI, but supposedly their DSL is even better since it fully compiles to native Rust code.