@lplr I just started using Figma to design the UI for my next plugins. It's mainly vector but you can drop images in there too.
It'll work fine with any of the assets from the aforementioned UI design services.
The main advantage for me is speed of iterations. You can easily nest and group things, and use constraints to keep things evenly spaced and shrink/grow as you modify your layout. You can't do this very easily in the HISE Interface Designer.
Once you're happy with the design, you can take the UI elements' position and sizes straight into HISE scripts that create the UI.
I know it's an extra piece of software to learn, but the flexibility of this way of designing is a winner for me. It keeps a nice separation between functional and visual design.
Additional bonus is I can share the progress of the visual design with someone else (stakeholder, collaborator, etc) without having to either share the HISE project or having to continually take new screenshots of the UI in HISE to email/message to them.
If you have a team it's even better. The UI designer can work on the visual design in parallel with the plugin/DSP developer, and as long as they have a central interface document - showing what parameters and displays the plugin exposes in the UI along with ranges, units, etc. - then combining them near the end of development should be cake.