It's hard to say since they added features over time. Hangouts launched in May 2013, then got one side of the MMS features working via Google Voice numbers in September 2014 http://www.droid-life.com/2014/09/10/mms-now-a-part-of-googl... Depending on your carrier, it might send the SMS recipient a URL to the content instead of a real MMS. And even now, there's no way to search messages from within the Hangouts app. You have to search from your gmail!
There was some other limitation for a while that kept me from moving. Group messages, or voicemail maybe? They did eventually fix it though, so I can't remember.
