In the command bar designer, you can define what happens when a command is activated - execute formula or JavaScript.

For visibility, you can only choose between 'Display' or 'Display with condition from formula'. This means that for simple buttons with more complicated display conditions, the entire button must be created using the XRM Toolbox, as the requirements in the visibility rule can only be implemented to a limited extent with Power FX.

This problem would be solved if the visibility could also be defined with Javascript.