It was also really nice when there was an iOS app named Talkatone that combined Google Voice calls, SMS, and voicemail into one. Circa 2012 that stopped working and Google rolled out separate Hangouts and Google Voice apps and nothing has been as good since.
In the meantime I became sickened by Google's ethics and behavior and have migrated all of those services to a combination of Fastmail and Anveo.
Also, you could do the following things which Voice doesn't support:
- Have multiple chats open (and be working on several responses) at the same time
- Star chats (nice workflow for remembering to respond later)
Although there were a couple things which, annoyingly, never worked right:
- Receiving contacts (vcards) from iOS.
- Starting a new group text (more than one recipient) from within Gmail. You could respond to an existing group text but to start a new one, I had to do it from the hangouts app on my phone.
I had some very minor flakiness with receiving certain SMSs with Fi/Hangouts, and various anecdotes on the internet suggested not using it for SMSs as a solution. For me, that didn't make a difference, but it's like they've given up on whatever complexities there are around unifying, and going back to two lowest-common-denominator siloed applications.
Another part of it is probably that Hangouts / google chat is a channel (network) that they completely own, whereas SMS is a generic, third party protocol, and history tells us that XMPP, MSN/Yahoo Messenger integration, Facebook RSS feeds etc can't last because each company wants to be the sole owner of a walled garden, and not a slice of a larger pie.