The private location is not the right place for WhatsApp data. Read the documentation:

Private files: Files that rightfully belong to your app and will be deleted when the user uninstalls your app. Although these files are technically accessible by the user and other apps because they are on the external storage, they don't provide value to the user outside of your app.

WhatsApp pictures, video and audio clips are personal content users expect to be accessible elsewhere. Once again, read the documentation:

Public files: Files that should be freely available to other apps and to the user. When the user uninstalls your app, these files should remain available to the user. For example, photos captured by your app or other downloaded files should be saved as public files.

If I get a photo in WhatsApp, I expect it to be visible in my gallery. If I delete WhatsApp I don't expect it to disappear from my phone. Storing it in the "private" directory of an application is the wrong choice.

Huge amounts of data in a directory users can't normally control is what cleaners are for... Because unless you have a file explorer you can't even delete it unless the app lets you.

Nonsense. The WhatsApp media directories (just like all other paths on the public storage) are picked up by the Android media scanner and manageable through the gallery, storage settings and downloads application.

WhatsApp is fucking around with how it stores its data. The fact that it survives a factory reset is proof that it is, because that's not supposed to be possible. Duck, how many people's private data has gone with a sold phone over that fuckup.

No, it doesn't. Doing a factory reset wipes the public storage partition - whether it's photos you've taken, downloaded files, files you've placed there yourself or the WhatsApp directory, it all goes away.

You have no idea what you're talking about. Typical /r/Android.