Worst of all it's easy for redundant syncs to happen. "iCloud backup" on default config backs up the entire device, including your Camera Roll. "My Photo Stream" also uploads every picture as well as "iCloud Photo Library". This turns 5GB of pictures into 15GB. (Edit: wrong info see post below)
The most fun I've had a few days ago with "WhatsApp". They introduced their own backup capability via "iCloud Drive" which also doubles their backup requirements if not configured properly. Default iCloud backup already backs up all of WhatsApp. WhatsApp's iCloud Drive backup then saves everything again. In my case WhatsApp backup got stuck and was indefinitely uploading the same file. Turning backup off in WhatsApp-settings did nothing. I had to rip out iCloud Drive access for the app to make it stop.
This was on my sisters device. Given that WhatsApp is heavily used for image sharing as well this meant that WhatsApp was (trying to) backup 2*2GB of data. Not possible at 52KB/s upload speed.
Bonus points: WhatsApp also saves all images to the Camera Roll so the image gets backed up once more.
Fun times.