What could make build progress stop at "Building assets for scene 0"?

Sometimes it takes a really long time to get past that message and finally continues building, sometimes it just hangs and shows the spinning rainbow cursor indefinitely until I force-quit Unity. Is this happening to anyone else? What do you do to fix it?

A bad shader. I figured it out by adding one asset at a time from the latest project to a copy of the last backup that finished building and when it got stuck again, there was my problem.

Nope its not a bad shader. Basically its the trees, I removed all the trees from my scene and it worked fine! So I guess the trees take ages to process

I had a perfectly flat terrain and this happend. I added a small mountain and it built successfully very fast. A flat terrain is causing the baking to hang. There you go.