US citizens and green card holders
US laws and IRS policy make it complicated for US citizens to maintain accounts and invest in other countries. Many banks and institutions now refuse to take on US clients due to the cost and risk of FATCA and similar legislation.
At time of writing, US citizens and green card holders cannot open accounts with Robo-Advisors in Japan.
They should avoid investing in foreign-domiciled mutual or index funds. Older US citizens may want to look at iDeCo as a way of reducing Japanese income tax by saving in cash, but for younger people this is unlikely to be a good strategy.
NISA accounts are a flexible way of reducing Japanese taxes on capital gains and dividends, but they would not be tax-exempt in the eyes of the IRS, and most brokers in Japan only allow US citizens to trade Japanese individual stocks.
There are two options that may simplify things for US citizens:
1. Interactive Brokers is an online broker service that is willing to deal with US citizens living outside the US. As they are an American company, reporting and tax compliance is greatly simplified.
2. American Citizens Abroad has a range of services and advice.