It uses too much space in phone's internal storage. I don't get it, why there is even an option for cloud backup when it clearly doesn't use that. If something is in the cloud why are you using internal storage. Google photos does a good job like backing up in cloud and clearing phone's storage and i can see all photos without wasting my storage, and when needed there's always an option to download. WhatsApp definitely needs to learn from Google how to use cloud storage.