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.