I like WhatsApp's features and UI. That said, it's really difficult to list the issues because, in a way, we are listing symptoms of the same cause: it's a centralized system colluding with the a centralized government.

Here are a few reasons off the top of my head :

1. It's heavily tied to phone numbers, making it easy to obtain the personal information of number owners.

2. They keep communication metadata.

3. They cooperate with law enforcement in countries without free speech rights. When they denied to cooperate here in Brazil, a judge blocked Whatsapp in the entire country. Therefore, if they want to do business in the country, they necessarily have to collude with authoritarian regimes to crush their opponents..

4. Meta integrating LLM everywhere, so it is kind hard to assess how exposed your personal data might be