T-Mobile service works pretty much as advertised in Japan (and China) on the T-Mobile One plan. The first time you connect in a region, you'll get a text from T-Mobile detailing the benefits.
It's basically minimum 2G so if something higher is available, the phone will connect to that so if you see 4G or LTE, it is still included in the plan. General data stuff, Facebook, Skype, Pokémon Go, etc all worked fine. I did not try YouTube off Wifi but videos on Facebook played pretty much normal. The AppStore was a bit sketchy so if you really need to install or update, find WiFi.
The only odd thing I noticed was sometimes the data connection took longer to activate but it eventually did.
Also, keep in mind WiFi is prevalent enough to make WiFi Calling easily achievable if you really need voice. Anything over WiFi acts just as if you were home. Note, you do need a T-Mobile branded phone to use WiFi Calling.