My experience is that the UI arrangement and widgets will change a lot during development so I definitely think it is a good idea to keep them as simple as possible at the beginning.
Call me old school but I’ve always found the Projucer JUCE component editor handy for quick mock-ups (the JUCE team is phasing it out though 
). When JUCE was born, this was the way to go but over the years most developers have developed their own UI designers. This requires a significant effort of development tough. Some create them from SVG files (I do), some create them programmatically (foleys_gui_magic, Introducing JIVE - the ultimate JUCE extension for building GUIs - Useful Tools and Components - JUCE) .
Now everybody is talking about webviews probably because it is cheaper to create UI’s with them (standard web front-end developers can be hired for that) however I still have to see an UI made with webviews which looks the same in mac/windows/linux, has good quality sliders/rotatory knobs and feels responsive. There have been comments on the forum mentioning audio companies that are already using them but I haven’t seen any code or examples (I’m genuinely curious about it, maybe somebody can share some examples?).