Starting a hangout often works for me, but I rarely get notification messages. Sometimes it takes half an hour or more to get a notification that was sent by someone in the same room. I switched to SMS for IMs, because people don't get the Hangouts messages reliably.
Hangouts could be good, but it's awful at the moment. I wish they had at least kept the XMPP integration. I strongly suspect that Google would be more successful if they went back to a more open approach. If they had backed Diaspora rather than creating Google Plus, it would now be huge. Google Plus didn't even have a decent API. I have a lot of ideas on how to salvage it, but no one at Google has ever responded to my suggestions.
In the list of unreliable messaging, Skype Android app has been nicely random in how it notifies me too. I can never know if someone talked or not. They keep changing the UI though, how touching.
ps: I believe all your points on xmpp/diaspora are right, and that's how Google got popular, bringing good product (mail, maps) for free. Not trying to invent a whole G paradigm because Jobs said 'focus' in an Apple keynote.
The Skype app for Android and Linux were so bad that I stopped using Skype completely. I thought that Hangouts was going to be a perfect replacement, but the notifications became too unreliable. If anyone from Google is reading my comments, and would like ideas on how to fix it, send me a message. I do want Hangouts to work. :)
Hangouts could be good, but it's awful at the moment. I wish they had at least kept the XMPP integration. I strongly suspect that Google would be more successful if they went back to a more open approach. If they had backed Diaspora rather than creating Google Plus, it would now be huge. Google Plus didn't even have a decent API. I have a lot of ideas on how to salvage it, but no one at Google has ever responded to my suggestions.