I guess as an analogy - it would be comparing a road bike to a mountain bike - both able to ride on the bitumen, but much better on the (purpose designed) road bike. There are bikes that can do both, but they don't perform as well on a road as a road bike, nor are they as good as a mountain bike.
The technology used to design a windsurf board (ie; stance, footstrap position, fin position, propulsion through the sail and specific weight transfer down from the mast-base through the harness) can't simply be altered to suit where the rig is disconnected and a paddle is used in lieu of a sail/rig. Similarly, a SUP is designed for a different stance, therefore the weight distribution is different. The propulsion is from a paddle being drawn through the water (or actually the paddle is supposed to remain static whilst the board is pulled through the water) creates a completely different design requirement, although they both have rocker, volume, rails etc.
Certain brands have marketed their craft as being able to perform well as 'windsups' (hybrid of SUP and windsurf boards), and I'm sure that they can be used for enjoying the options...however as 'stand-alone' craft, the sacrifices that are made to the designs so that they can cross-over, has to detract from the performance - IMO. I like to rig up my 11'4" Nalu on occasion, and I'm glad I sourced a SUP with the option, but if I want to go windsurfing, the SUP stays in the shed. I started SUPing on a 165lt WS board...that got me interested, but it was never going to last as a SUP.