I went digging and found a reasonable review for Chromecast Audio in a multiroom setup. Reading it through, it's easy to see how you can do a lot. Pair it with an amp and speakers. Pair it with a cheap (or not so cheap) pair of powered speakers. There are even a few vendors selling powered speakers with Chromecast Audio support built in (not reviewed in that article, but still there). Here's a second review that points to some hiccups, but also notes that Apple's Airplay also isn't working so well these days.

The biggest downside to Chromecast Audio, and it's quite a bummer, is that you can't do the multiroom audio thing through a video Chromecast stick. That's annoying because you might want to use an existing television as a audio streaming source. Presumably, it's just a matter of software for them to do this, making it inexplicable that they're not doing it. Maybe, maybe, it's something weird with the way licensing works for HDMI.