its not secure. if you have facebook app and whatsapp installed. the whatsapp can share data with the facebook app and upload it to their servers. messages on your phone are not encrypted. they are only encrypted during sending.
A good workaround for Android is installing "Friendly" in place of official FB app. Not sure if there's an iOS version too.
Also, if you are on android and do a Gdrive backup, all the WA logs are in the clear on googles server. They will gladly pass these logs to authorities on with the appropriate paperwork.