Any discussion about Green Cards or permanent US residency should always mention eventual exit tax[1]: "...to deter tax avoidance by abandonment of citizenship, the United States imposes an expatriation tax on some of those who give up U.S. citizenship. The tax also applies to green-card holders who abandon U.S. residency after having held a green card for at least 8 of the last 15 tax years."

[1] https://en.wikipedia.org/wiki/Expatriation_tax#United_States