My main critique is just that, within the Mastodon side of the Fediverse, the design is highly misleading about what the feature does. It resembles a normal DM feature, but the message addressing is purely handled by mentions in the message body.

Basically, it's an antipattern, causing people to accidentally mention other people in what's assumed to be a Direct Message. It's less about privacy, more about poor telegraphing of side-effects.