But also there are a few which are remarkable: the 3 synths from Madrona labs, the prosaic and convenient UI from Valhalla's plugins, up to the complex patchbay interface of something like VCV Rack, for instance.
VSTs balance a number of interesting perspectives in their UI. They are facing a very challenging domain (audio synthesis, analysis, manipulation in creative and often real-time interaction), have a complex user base (ranging from pros who really would prefer to be using their actual rack effects, to audiophiles who are pretending they've got a garage of classic synths, to modern electronic musicians who are digital natives), and have essentially zero strong UI non-skeumorphic conventions (knobs, sliders, presets, A/B switches, modules).
It's totally the wild west of UI.