WeChat

The awfullest IM app I've ever used:

Only support one phone and one computer login at the same time.
Unbelievable large storage use (~50 to 100 GB)
Message history will not sync on different devices
Each time you share a file WeChat will make a separate copy of that file in your phone
Ad everywhere
Terrible back-end battery consumption
"Sensitive" contents will directly disappear in the chat
You may be banned forever for unknown reason
......