We have have implemented externalizer which works fine for all the links. However src property of images are not externalized. We do not conditionally externalize links.
For pages or image components that render within AEM it isn't much of a problem. However this becomes an issue with content fragments. The main reason we create a content fragment is to expose it to outside apps.
We have a content fragment which is exposed via the assets api to the react app. If we add and image into the Rich text editor, that image is not externalized making it totally useless to react app.