Not quite. When adopting the SD card, you have to choose ONE location for user data (that is data every app has access to and which is also accessible via USB, like photos, videos, downloads). This can be either the phone’s internal storage or the SD card. But you can move apps and their private data between both locations. Also, some apps allow you to choose a storage location for their data even when using adoptable storage (e. g. Google Play Music).
It really depends on the app. If it doesn’t fully support the Android 6 storage API there is nothing you can do (except complain to the app developer).
I use the SD card as external storage and the following works great for me:
Camera saves all pictures and videos to the SD card.
Spotify Downloads Music to the SD Card
Pocket Casts Downloads podcasts to the card
My completed mp3 archive of pre spotify times is on the card.
So, I can hardly say no application uses it. Almost all data hungry apps I use can save their media files to the card. And I still have a more then 12GB free phone storage. This is way I always recommend to use the SD card as media storage.