From a cartographic perspective, showing all layers in the legend isn't usually necessary. It can cause unnecessary clutter and confusion, especially for thematic maps that may have reference information, for layers used as visual "tricks," or for maps where items are communicated intuitively through labeling and/or symbology.
It would help me a lot of the legend widget had customization capabilities similar to ArcMap/Pro.
For example: in this map, I'd like to nix the MPA layer or at least move it below the others.
