Apparently, some users have reported that messages which have been received by Voice too, are slowly but surely appearing on Hangouts without issue, while replies aren’t exactly the most stable of options at this point in time since they do not get sent out prim and proper, which means that making the jump would be pointless right now.
Apart from that, if you were to make this switch, it will result in your Google Voice extension in Chrome to cease working, which translates to your inability to check as well as respond to texts on your computer. Last but not least, it is impossible to select the number you would use whenever you respond to a text, as you will remain locked in to whichever number that it hailed from. Hopefully all of these growing pains will be sorted out sooner rather than later!