I just returned from a week in Tokyo and Hiroshima. My unlocked iPhone X worked just fine everywhere I traveled. In fact, on the approach to Narita airport, prior to landing, I got the welcome messages from T-Mobile on my iPhone and iWatch. I had downloaded and set up one of the local transportation cards (Suica) and it worked just fine "tapping in and out" at train and subway stations and at local food marts.
Even being restricted to "2G like speeds" was never an issue. WIFI is ubiquitous even if you perceive a download problem.