Blender has seriously complex UI needs. No idea whether they use a reactive architecture but (claimed) raw performance is impressive.

https://github.com/ppmpreetham/BUI

Apart from the main 3D viewport Blender's UI need don't look that complex. Most of its UI are panels with simple layouts and buttons/sliders/checkboxes. And I bet they don't use their UI library for the main 3D viewport.