You have already answered your question. It all depends... We were grappling with what do use.
In general, we use the Qt Designer for following type of work.any reusable UI component customising the existing widgets e.g applying stylesheets Specifying some generic property values to existing widgets which can be used everywhere
After all this we need to assemble all these components and objects make the complex UI. While making this we create the object dynamically and use it appropriately.
Many times I see people are using designer to design the entire UI. When they launch the app, whole complex UI is created. E.g I have 5 screens. Each screen has internally another 10 screens. Each screen internally has 20 object. Whole 51020 UI done using the designer. This is huge problem for designing.
In summary we use Designer for Component creation. Finally assembly is of components is done manually ie. on demand.