I've never used Google Voice, but I do use Google Fi, and they offer unlimited calls and sms in what looks like every country on the planet. What's the advantage of Voice for the traveling caller?
The biggest problem I've had with Fi is that I destroed my phone while traveling, and I couldn't find a phone internationally with which I could activate Fi while traveling abroad. I could only associate the new phone w/ Fi once I returned to the USA.
I use Google Voice as my phone for nearly everything, unless it doesn't work.