In Canada, our retirement system is built upon three pillars, CPP/QPP/OAS (similar to US Social Security), RRSP (similar to US 401k), and TFSA (similar to US Roth IRA, but more inclusive, as every resident can use it regardless of income level). TFSA is arguably more important and a higher priority for retirement savings than RRSP in Canada. However, the US does not recognize TFSA as a retirement scheme. So if you are a dual American-Canadian citizen, the financial advice is not to use TFSA, as a) it won't be tax-sheltered, you will have to pay taxes to the US, and b) the paperwork required is a huge headache.
Every country has something that they don't tax and US will. Because that country does not tax the thing, there is no tax credit to offset US taxes payable and you may end up paying a lot. The thing may be a retirement savings (Canada) or capital gains (Switzerland) or a myriad of things other than employment income.