The problem comes from federation. You never know where your messages are synced to + what will happen if instances are defederated. Matrix might become something really cool, if it spends 1-2 years solely on security. Otherwise… it’s just nothing more than an epic (and misleading) name + some IRC legacy vibes.
Here’s a great and beginner-friendly article on the topic of secure (and private) messaging and why Matrix currently doesn’t fit there: https://proton.me/blog/whatsapp-alternatives