WhatsApp is a simple yet comprehensive app that can be used on iPhones, Android and Windows phones. This, combined with end-to-end encryption, makes it very popular with many families, associations and companies. They run their internal communications through WhatsApp. The impression is given that WhatsApp is your safest way of communicating.

Since 2014, the content of all messages within WhatsApp has been encrypted, so that only sender and receiver can access the content. However, the “meta-data” of messages and conversations are simply shared with Facebook. What they do with it is completely unclear.

WhatsApp requires you to provide access to your entire contact list. This allows them to create a complete overview of your social network based on name and mobile phone number. WhatsApp will also store information about a message, such as which numbers received it, when, and so on. This is how they refine their knowledge of your social contacts.
More information: https://www.researchgate.net/publication/312778290_WhatsApp_security_and_role_of_metadata_in_preserving_privacy

Recently, the founder of WhatsApp announced that he will leave the parent company Facebook, presumably because the visions on privacy are too different. In an update to its Terms of Service and Privacy Policy, WhatsApp says it wants to work more with Facebook in the future. For now, they would not share profile data, but this is the plan anyway.

Data sharing: We do not currently share account information to improve your experience with products and ads on Facebook. As we’ve said in the past, we want to work more closely with other Facebook companies in the future and will update you when we’ve worked out our plans.

After reading all this, I am less and less a fan of WhatsApp as a safe way to communicate.
On the other hand, it is a useful app for family members, friends and acquaintances. Convincing them to switch apps won’t be easy.

An alternative might be to use Signal, but then again, they have their own issues.