Really, I don't understand the hate Hangouts keeps receiving...
Hangouts is horrible at keeping chats synchronized. I will sometimes find messages sitting in the Chats folder in gmail that I never saw on my phone and as a result never responded to.
Having SMS in Hangouts is kind of nice, except that it is just window dressing since you can't see those messages on any other Hangout instance other than on that one device.
Also since Hangouts relies upon your network connection it is pretty unreliable as an "instant" messaging system. I find that I can get someone's attention much more quickly via SMS than a Hangout message, which in some cases have taken hours to deliver.