Project Fi is already good for international travelers. The flat rate of $10 per gigabyte (the same price for using data at home) is better than what you typically get charged by a US carrier when you're out of the country. But truly "pro" travelers with capable phones still often prefer to get a local SIM card for one simple reason: speed.
Roaming data is almost always slow. It's typically capped at around 3G speeds, meaning you get none of the modern 4G LTE you've become accustomed to in your native land. 3G is a godsend for someone just trying to look up directions, read a Wikipedia article, or send a Facebook Message, but it's not good for much else.
But since Project Fi isn't a single network, but a collection of networks — it's already powered by Sprint, T-Mobile, and just recently added US Cellular — it has a leg up. Instead of negotiating another roaming deal, Fi just added another network: Three.
Three covers the United Kingdom, along with Austria, Denmark, Hong Kong, Ireland, Italy, and Sweden. This should give Fi speeds "10-20X faster than before," claims Google. But unless there's something Google isn't telling us, those speed bumps should only apply to where Three has coverage — not all 135+ countries Fi covers internationally.