Why I can't see Viber Videos folder in Windows Explorer (on Huawei P9)
I think that folder has some kind of system protection.

However, there is a way to see images and videos (at least on Huawei P8): Open Files app (Huawei's default file explorer) and go to /viber/media and rename it's child folders (Viber Images and Viber Videos) and you'll be able to see them in Windows 10 Explorer.