Below is my personal experience with the Teracube phone and Google Fi. Hopefully, others have fared better.
I switched to Google Fi with the purchase of a Google Pixel XL several years ago - no problems. I then purchased the Teracube, swapped SIM cards and began the Google Fi activation. However during activation, I received an error code. After multiple attempts, the phone finally activated.
All was good for a while, until I noticed the battery drained quickly. This issue was identified and resolved with a software update for most Teracube users. My phone however, continued to experience rapid battery drain and felt warm to the touch. It was eventually discovered that the Google Fi app is the culprit (a Google search identified this issue for other brand phones as well). A process hangs and consumes the CPU, until the phone is rebooted. After a while, the process hangs again and this repeats continuously. I performed numerous factory resets, but to no avail.
Removing the Google Fi app did resolve the high processor use. However, the functionality of the phone suffered. I constantly received notifications to activate my SIM card, text messages ended with random characters (example ~+DXjVwAOoy+4Qwg), and I could not enable WiFi calling.
I received a new SIM card from Google and experienced the same problems. Downloading the Google Fi app and activating the SIM card lead to error codes once again. Once activated, there again was the high processor usage and draining of the battery.
I finally gave up and changed my carrier to T-Mobile. All is good once again. I would like to add, the Teracube phone has exceeded my expectations.