Qt Design Studio is buggy and heavyweight. It lacks integration with Visual Studio, has a bad integration with Qt Creator. It doesn't produce clean .qml files. Qt Designer (the one used for Qt Widgets) is better in those regards.

Anyhow, this might be a controversial opinion, but the best experience I've had with WYSIWYG UI designers was with Netbeans Swing Editor and VS Windows Forms Editor. The IDE integration was seamless.