Just my personal experience with a Nexus 5 on T-Mobile -

Last year I went to India on T-Mobile with my Samsung Galaxy S4. Was totally fine, sometimes sluggish, but maps and Instagram were accessible. I was also able to use it fine in Canada (not a good data point, but still).

This past year I went to the same parts of India with a Nexus 5 and it was a disaster. I couldn't connect to any carriers. My phone constantly errored with a "No SIM card found" error. I had to restart my phone several times a day. My travel companions on their iPhones/T-Mobile had no issues. I also had issues in Switzerland on my Nexus 5. No bueno.

I have very little faith in Project Fi if my Nexus 5 was an example of what is to come.