Foreign law can be useful in understanding how the rest of the world views their own statutes and constitutions. If good reasoning is used on similar cases to American ones, then there is no harm in referring to those decisions. What is harmful, however--and here's where it gets tricky--is the notion that foreign decisions can be binding in America. They cannot and should not be. As Chief Justice Roberts is quoted as saying:
If we’re relying on a decision from a German judge about what our Constitution means, no president accountable to the people appointed that judge and no Senate accountable to the people confirmed that judge, and yet he’s playing a role in shaping the law that binds the people in this country.
That would be a bad outcome. Foreign law should be looked at for the value of the argument, like any other argument, but not to be binding in any way on Americans.