Rebuild command window has a preview button. The preview does not work for surfaces and curves. This is a minor bug.
It seems to be working here on a simple test and I couldn’t find any open YT issues for rebuild but perhaps some are hidden from public view.
Could you send a curve or surface and the values you are using to rebuild.
There used to be a cap for drawing the preview of complex curves / surfaces. I’m not sure about the current status and if things have been done in this area for Rhino 6.
The preview in Rebuild works for me for both curves and surfaces. However it can be difficult to see if the rebuilt curve/surface is close to the original because the original stays highlighted.