It’s also hard for any couple when they enter into a transatlantic relationship as homesickness can apply and a yearning for the security of your roots and home country. I can easily understand how MM found it hard to encompass the UK plus the RF, but equally well I can’t help but think it’s going to be hard for PH to adjust to a permanent future in America and leaving behind the basic safety net of his own family with all that entails- both good and bad.
I'm in an international marriage myself, as are the majority of my friends (I live outside the UK).
IMs are tough and require work, sacrifices and realistic expectations.
They tend to work best when the non-native spouse (the one who is residing in other spouse's country) maintains reasonably strong and solid links with their own country, like having some local friends who also come from the same country, maintaining strong relations with friends and extended family members back in their home country, and so on.
A "Our Little Family" mentality where friends and family from the "other country" get devalued and cast aside is unlikely to work long-term, IME.
The Royal Family did a poor job of integrating Meghan (although in part, H and M made some poor choices as well in this regard). Over in the States, Harry does seem to have alienated a really high % of people in his former pre-Meghan existence (not just his family but also his army buddies and so on), and I'm not sure this has been a great move.