With George Romney it was a legitimate challenge. George Romney's parents were not under the jurisdiction of the United States at the time of his birth. They were in Mexico escaping the laws of the United States in regard to Polygamy.
McCain was born in a US Territory that Carter later gave away.
McCain was also born to American parents who were in the military. We have hundreds of US citizens born all over to world every year to deployed military family members. How many realize that McCain’s father was an Admiral, and that his grandfather also was an Admiral. He comes from a military family.
In the case of George Romney (Mitt’s dad), IIRC, George’s father’s citizenship seemed clearly to be Mexican. George’s parents clearly had emigrated to Mexico prior to George’s birth. George’s mother’s citizenship was dependent on the her own emigration to Mexico. The question appeared to be whether or not it was prior to Utah’s statehood, the status of those who emigrate from territories instead of states, etc.
They appeared to be real questions, but they were made moot by George’s dropping out of the presidential campaign. I don’t think anyone ever demanded an official ruling.