I'd be interested to know the basis for this advice, as it hasn’t been my experience that the trámites have been any more complicated with a foreign marriage. All we had to do was to get a constancia de matrimonio when moved to Mexico which basically consists of a certificate showing that your foreign marriage has been entered into the Mexican Registro Civil, and then any time you’re asked for a marriage certificate (acta de matrimonio), you produce the constancia and it counts because the marriage has been registered in Mexico.

Agreed. Instead of having to renew every year forever, as in the case of an FM3, after five years of annually renewing your FM2, you are inmigrado and you no longer have to do any renewals. (That is if you don’t nationalize first. If you plan to nationalize as soon as possible, the FM2 enables you to do so after 2 years.)