The main concern isn't just unknown parties getting to read your benign chitchat.
You may not care about the metadata WhatsApp collects about you. Until it shares it with third parties who target you and then, too late. This doesn't just mean a barrage of junk mail.
A separate, and probably worse, threat is from receiving pictures with embedded viruses. Pictures on WhatsApp are downloaded to your phone automatically unless you manually turn off this feature. Many people leave the default because they either don't know how to turn it off or they quite like having pictures from mates, family, etc. saved automatically.
Hackers exploit this to send you WhatsApp images with embedded virus files. These download as soon as you view them, the virus is activated and the hacker has immediate access to your phone.
Once they have access to your friends and families' contact details, they have open invitation for any depths of malicious behaviour to extort money (or just to embarrass you).
These are only a couple of the security risks inherent in WhatsApp which other programs, e.g Signal, don't have. There are many more.
It doesn't mean you shouldn't use it if you want to, but at least go into it with open eyes.