it's not just about taxes. US Citizens who live overseas can't invest in what are effectively foreign mutual funds (PFIC in IRS parlance). While retirement plans are generally exempted if there's a tax treaty, that doesn't help other forms of money one might get.

Your local government gives your child a grant of money each month to be invested in a choice of funds? You're a US citizen? sorry, you're out of luck, this makes doing taxes effectively very very difficult.