Fantastic use of ACF. Plus a suggestion to improve it.

Fantastic! This fixes one of the biggest problems with using WordPress for client sites.. letting them easily set the widget-area for each page, rather than hardcoding it into templates. I know other plugins add a UI to each Widget, to set which pages/posts it appears on. But it’s more intuitive for clients to select a widget-area while on a page.

One suggestion — if the ui were checkboxes (rather than a dropdown), multiple widget-areas could be added to a page, rather than just one. And one step better, would be to enable the checkboxes to be sortable by dragging.