One example: The Google search box requires the search results to be shown on top of the page being viewed. This is not possible with an iframe. Unless you make the iframe very large which messes up the design of the page.
Great to see the shared header and footer in the latest release 1.30. However without being able to add code for analytics and cookie compliance it is not yet possible to use the App Builder for real websites.
Any update on this? Unfortunately despite an impressive and growing feature set the App Builder is next to useless for public facing applications without being able to add custom HTML/JS.
This feature is on our roadmap, and is not yet implemented. You can monitor its progress on Gitlab. When the development team next reconvenes, I will update the Gitlab issue with a more accurate timeframe, I hope this helps.