For theme authors to join the FSE bandwagon, I think it is necessary to have a clear statement of what kinds of features the Site Editor will provide exactly. What degree of flexibility it will have.

For example: will it be possible to style a complete website with it, considering grids, responsive design, custom icons, animations and so on? Or should I need an additional block plugin, with tons of custom controls to do this?

I saw some hints here and there in Github issues, but at the moment there is no clear picture.

See, if we need to rely on three or four plugins to create a custom theme, it’s better to keep using just one page builder plug-n, isn’t it?

Without the flexibility to deliver the type of layout that a page builder can, what's the point?