In the past few years we have made use of TM's international roaming and about 13 countries. Last month, we were in Myanmar (fka Burma) for a couple weeks. That is one of the few countries not on TM's list.
Up until 2 1/2 years ago, when the military was in control, very few people had cell phones. Now everyone does! Besides the government carrier, there are two private carriers. We covered a lot of the country, and top grades was excellent everywhere. Even though one carrier, the one we used, has 4G service almost everywhere they serve, service is slow, by our standards, because almost every one has to use the cell carriers to access the Internet. All the systems tend to be overloaded.
Rates are very very cheap. A gigabyte of data is less than a dollar. The Sim card cost $1.50. I put $20 on the Sim card and "assigned" most of that credit towards data since we were only going to make a few phone calls within the country. I put the Sim card into an extra phone, set up the hotspot feature on it, and we set our regular TM phones to Wi-Fi calling only. We shut off data roaming. In effect, that gave us access to Wi-Fi wherever we were.