WhatsApp doesn't have an open source client so verification is difficult. However, if someone were able to break the encryption, I'm sure it'd be in the headlines of most newspapers.
One exception is WhatsApp business: I don't know the details, but Facebook offers a service where they will do some chat automation for your business which means they must receive the keys.
In terms of security: key changes are automatically accepted. They are hidden by default, but by toggling a setting every time a user updates their keys, a message will be introduced into the chat. QR code key validation has been in the app for years now, though I doubt many users are using the feature.