Differing rules of what’s a short-term vs long term capital gain.
E.g.: Canada doesn’t tax capital gains on your Primary residence when you sell it, but US does.
And buying a non-US ETF can create another tax mess since you’ve now bought into a multibillion dollar trust that doesn’t file with the US gov.
The US doesn’t see your Canadian Tax-free savings account as tax free, so now you have to keep track of every transaction in there.
https://www.investopedia.com/ask/answers/06/capitalgainhomes...
It's a lot of money, but if you bought a house in Canada in Toronto/Vancouver 30 years ago, you'd be paying US CGT on its sale.
And can a US-citizen/non-US-citizen couple still file jointly?
I’ll be first in line to complain about complicated taxes. I file three income tax returns, thanks to the fact I live in a jurisdiction with local income taxes as well as state and federal.
For example: federal student aid doesn't require US residency.
https://studentaid.ed.gov/sa/types/international
Also, the taxes that the US imposes on citizens overseas has some pretty significant exceptions. US citizens can deduct over $100,000 worth of their foreign-earned income and part of their housing expenses.