The Qt stack does not have support for mimetype handling for files (e.g. I want to open any text/html file). You need KDE for that.
Qt/KDE requires you to generate moc files (preprocessing the C++ code) which can make it harder to maintain if not using CMake or the Qt project format.
Qt broke the behaviour of QAudioDeviceInfo by moving it to a different package (QtMultimedia vs QtMultimediaKit). There have been times where Qt has not shipped a pc package (esp. in the package QAudioDevice moved to) which makes it difficult to add that package on build systems other than the Qt build system.
Qt does not support using gettext for translation.
Qt is in a transition from QtWidgets to QtQuick which (like Gtk3 themes) is currently a moving target.