What was really horrible and downright ATROCIOUS was the Messenger (SMS) integration into Hangouts. I was jumping for joy when they freed SMS from Hangouts again recently.
EDIT: I see now, I think my experience is different from others in that I only use my Google Voice number for everything. Doing SMS to/from your actual phone number inside of hangouts is definitely a disaster. If that were my use case I'd be using the Messenger app. If you're exclusively using your Google Voice number, though, the integration is pretty straightforward.