Data point... Google Fi worked swimmingly in South Korea. A few dead zones on inner Jeju Island as well as spots along the east coast between Busan and Sokcho; and the mountainous regions between the east coast and Seoul.

The day before we left the US, I switched from Simply Unlimited to Unlimited Plus in the app. It informed me that I would be upcharged an extra $42 for the rest of the month. Upon landing back in the US, I switched back to Simply Unlimited, and I was informed I would be switched at the start of the next billing cycle. That extra $10.50/person was well worth not having to hassle with buying four eSIMs and setting up four phones. The hassle might be worth it if we were overseas across two billing cycles.