Japan has excellent coverage almost everywhere tourists go, but network partnerships vary by provider — some ride on NTT Docomo, others on SoftBank, and that affects rural and in-tunnel performance more than big-city performance.
What to prioritize
For a 7-10 day trip covering Tokyo, Kyoto, and Osaka, a 5-10GB plan at 30-day validity comfortably covers maps, translation apps, and social media without throttling.