If they're for actual humans, even in the best case you're vulnerable to phishing, also you are a perpetual risk because you know these passwords (or a password equivalent) so an adversary might steal your passwords (e.g. from a backup, logs, test systems, ...) and now they can impersonate all users.
It's almost certainly safer than letting users pick their own passwords, but it's less protected than, say, a Google user who set up 2-step, and much less than if they went with Advanced Protection and thus can't get phished or impersonated.
Now we are moving away from this business model and will give more priority for custom domain e-mail.
Soon we are launching new pricing page with feature list of free/premium personal mailboxes and custom domain mailboxes