It’s a fantastic tool to prototype ideas, but that’s as far as I’d go with it nowadays.
It depends on the design and aesthetic you’re after too; as tempting as it is to just start playing around, I’d recommend spending some time to really figure out what kind of aesthetic you’re after for your plugin as it will streamline the whole process and help you justify design decisions and the tools you’ll be using.
I personally design my sliders in Inkscape and use Projucer’s ‘SVG Path Converter’ tool to generate the code, which I then throw into my custom LookAndFeel. I then scale, position and rotate them as needed. My plugins have a flat design so this works fine for me.
Using SVGs will reduce scaling issues and if you draw your sliders using JUCE paths as shown in the tutorials, you can eventually create some really dynamic and creative interface elements which can improve the UI/UX experience for the user.
Some projects require 3D modelling/animation software to accurately simulate lighting, shadows, texture and perspective of knobs and buttons. It’s overkill for most projects but it has its place.