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.

Can it even do basic forms by dragging and dropping of components?

I recently tried and failed. It was either doing absolute positioning or failing completely - for example dropping components onto grid would just place them into cell 0,0. There was no way to design form with mouse.

I used a lot of different UI designers in the past and all of them were heaps and bounds better.

As sibling points out NetBeans Swing designer (with Matisse/GroupLayout) is the best, along with VS WinForms (although these tend to be non-scalabale)