Google Voice finally starts integrating with Hangouts, but it's a rocky start

Some users have been seeing the dialogue in Hangouts to migrate their Google Voice texts and voicemail into Hangouts, but the issues present mean we'd recommend you not switch over just yet. First, while messages received by Voice are making their way into Hangouts without issue, replies are not sending properly right now, making the switch a bit pointless. Second, making this switch will cause your Google Voice extension in Chrome to stop working, meaning you may not be able to check and respond to texts on your PC. Lastly, there is no way to choose which number you use when responding to a text, they just lock you in to whichever number it came from. This means if you want to reply to a Voice SMS with a picture message, you'll have to back out and start a new message with your carrier number (because Voice still can't figure out MMS). 

Of course, all of these issues will be sorted out eventually, and these bugs are exactly why Google does a slow rollout of features rather than turning it on for everyone. We'll be keeping an eye on it to see if this is really the start of the marriage between Voice and Hangouts, or if it is still just more testing, and we'll keep you updated. 

source: Android Police