The documentation for CoreMainMenuDelegate states "Main Menu plugins are always displayed in the "More" tab, they cannot be displayed as tabs in the bottom bar."  However, in some cases the opposite is true.

If you disable some of the items from the main menu in your Moodle site's Mobile Features settings (We have everything except for "My websites" and "Web page" disabled), and have a plugin which adds an item to the main menu, it will always display in the bottom bar.  Changing the priority of the handler will change the position of the icon, but only within the bottom bar - there is no way to make it display in the More tab as described unless you already have 5 icons displayed in the bar.

This is problematic as the behaviour of your handler's page has subtle differences depending on whether its displayed in the bottom bar or the More tab.  For example, the label is not displayed with the icon in the bottom bar, which you may want, and the "Back" behaviour is not the same in both cases.

From our point of view, this should either work as described in the documentation (never display custom items in the bottom bar), or there should be an option in the handler to display in the bottom bar if possible, or always display in the More tab.

In addition, even when using the workaround above, we had problems with the priority setting to control our item's position within the 'More' menu. Regardless of priority setting, the item appeared at the top of the menu.