I have been using the above tool and Excel 2013 with mixed success. The good news is that it eventually works, the bad being that when it doesn't the following horrible things happen when modifications inside of the Editor are made:
Question Is the Editor the state of the art for customizing the ribbon and adding context menus. I realize the price is right but would gladly pay for a smoother and more reliable dev experience
The CustomUI editor doesn't affect modules in the workbook - but if you do make changes to the workbook while it's opened in the UI editor, then yes you may lose changes.
Also, you can set Excel to warn you of UI errors which will at least assist you in working out why your tabs/controls aren't appearing.