Siri is even more limited. Siri works with Maps, iMessage, Phone, Apple Music, the Apple podcast app, and that's about it. You can use Siri to launch and pause Spotify (my preferred streaming music service), but if you want an artist, album, or playlist, you have to select it by hand. You can do that from the head-unit's screen, but Siri won't help you make selections.
As for messaging apps, only iMessage need apply. While Apple pitches CarPlay Smart Box as the ultimate copilot, there's no Slack, no Hangouts, or any other messaging app. So, if you're driving along and work wants to reach you with a non-iMessage app, you have to pull over to the side of the road. None of the wonderful integration available for iMessage is provided for any other app.