I've used Google FI quite a bit internationally. I've used it in about 10 countries and the only one I had any trouble in was Hungary (and I blame my phone at the time since my sister had FI there as well and her phone worked fine). Generally when I've crossed borders it's reconfigured to be on the local network with a couple of minutes. Data rates are the same internationally. I rarely make voice calls so I can't rate that reliability but data and text have worked well for me.
I end up bouncing around on WiFi or cell using data per the apps LINE and HANGOUTS (soon to die) if i have want to limit my use of GoogleFI cell. LINE is preferred for me. Just a tip for those of you that travel its .01 per minute.