Only if they're set up properly, which is quite the gamble. I was recently in a hotel and I listed all the chromecast devices throughout the entire hotel. I could see what everyone was watching and if I was a lesser person I could have controlled their TVs or changed what they were watching.

What about device like those Chromecasts which don't even have firewalls? The only real solution would be to bring your own hardware firewall / access point and connect it as a client off the hotel wifi. Who is really going to do that?