I use ctrl-G and option-G on my Mac for “hide rest of model” and “hide similar components”. I did that because G (or shift-command-G) and command-G are already associated with “create component” and “create group”. These seemed like related commands, as opposed to “H” which as you note invokes the move (aka “Hand”) Tool.
On the Mac, most command-key chords are pre-assigned to menu items or OS functions and can’t be used for keyboard shortcuts in SketchUp. They are hard-compiled when the code is built, and are intercepted by the OS and menu system before they ever reach the keyboard shortcut system.
The handling of the shift key depends on the currently active Tool. On many Tools it is the modifier key to vary the action of the Tool. That usage takes priority over your keyboard shortcut, so result can vary depending on what you are doing. So a quirk, but not a bug.