I would just be careful if any of the assets in the Japanese accounts would be attributable to your spouse for whatever reason. If they are, I suspect you'll need to worry about
FBAR and
FATCA. It's conceivable that certain of your assets could be attributed to your spouse (even if he/she doesn't work) to the extent that they came to you after you got married.
Furthermore, J401(k) and NISA (
according to Bogleheads) not recognized as being tax-preferred by the IRS, so your spouse would be liable for any US taxes on such accounts as if they were ordinary taxable accounts.
Note that I am not an expert but just wanted to point out some potential topics to research.