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.
I'm not sure what you mean by freed SMS from Hangouts - once the two are merged you send and receive SMS in the Hangouts interface, as far as I know that hasn't changed. (That's how I do it today.) It's pretty simple, there's a dropdown menu next to the message input box that lets you select between Hangouts or SMS (if you have a phone number attached to that contact.) Works the same on mobile or in the browser. I remember there was some confusion around that when they initially added it but for the past few months it's been pretty straightforward and reliable.
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.
Messenger has been in the Play Store for at least a year or two now, and you could always designate it as your SMS app. It was only recently that they started nudging users to switch to it from Hangouts (which is funny considering that they nudged everyone to use Hangouts for SMS not that long ago).
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.