I've been in Thailand 5 times over the past year and the roaming has gone hill over that time. SMS largely does not work reliably, data connectivity drops out, a high-speed data pass doesn't always guarantee even usable speeds, etc. The standard speed flirts with being so slow that it exposes implementation issues in apps themselves -- such as timeouts that are too conservative etc.

A high-speed data pass has worked much much better in Europe over this same period.

I'm on an iPhone 8 Plus (also have used 7 Plus over this period).

In Thailand I think I paid DTAC 200-300 Baht for 1.5GB data for 30 days. Compare with $20 for 1GB/10 days with a TMO high-speed data pass. Also, DTAC has an app that works, and with which it is possible to top up when not in Thailand, thus extending the SIM lifetime. Not having my USA phone number can be a PITA, but I prefer reliability of a local SIM card in Thailand. Maybe this will change, but I'm not holding my breath.