I edited my comment, since 70% tax rate is actually possible depending on the tax rate of the country you're living and whether they have a tax treaty with the US or not.

You’d still be able to credit foreign taxes paid against any US tax owing, on income anyway, treaty or not.

Where you get screwed is when countries have different philosophies on things. E.g. Canada doesn’t tax gains on your home, but you can’t write off mortgage interest on your taxes. Meanwhile USA taxes gains on your home but lets you deduct mortgage interest.

If you’re a dual citizen living in Canada, you pay both.

Or if you live in a country with predominantly sales taxes and no income taxes, well, now you get to pay lots of sales taxes and US income taxes.

Not only the US, but if you've ever claimed residence in California, they might decide to tax you for foreign income as well (if you accidentally leave any part of your life there behind when you move on, for example, leave a car registered there, or keep medical records with your doctor, even visit family there too often)! So add another potential 14% for high earners.