Twitter DMs are… ok? You have some level of control over them (on or off for randos) and are, speaking for myself, decently spam moderated. The value I find in them is that people tend to check them, and if I’m trying to reach someone I know from Twitter, it’s a natural choice.
But they aren’t as good as email. If Twitter feels shakey right now as far as even continuing to exist and function correctly, that means that conversational portal’s trust is also at risk. I’ve never felt a shake like that with email. Federated, and all. Not to mention the real privacy of them is questionable, search is limited, attachments are limited, etc.
The real Direct Message is just an email.