Nebula is using a GPU viewport. Am I then limited by GPU memory? Absolutely not. When GPU maximum memory is reached, the realtime viewport won't render the extra entities causing it. All the scene will still be available for CPU rendering.
How to rename entities inside the Asset Panel? In Nebula renaming works like in the Windows explorer, using a second click on the text when the entity is selected.
How can I use spherical HDRI maps? At the moment Nebula has support of traditional cube maps. Please see the environment modifier. You can find great tools online to convert spherical to cube maps.
I find the light display rectangles either too small or too big. What to do? The size of the rectangles can be edited via the billboard settings of the menu bar.
Transparent, perfectly thin, non-closed geometries and with various normals are not supported. When used they can generate artifacts.