Why?
For instance: http://www.information-age.com/whatsapp-can-read-users-encry...
If there are other insecurities, please post them here - I haven’t heard of others than this one.
And this: http://www.businessinsider.com/whatsapp-security-2014-2?r=US...
The fact that it is owned by Facebook is enough to make me stay away.
Besides that I would never touch WhatsApp. The fact that they hand your phone number over to facebook is one of many reasons to stay far away of this messenger.