I've been a very heavy user of international airlines' frequent flyer programs, but there are issues with using them if/when primarily flying US carriers or wanting to redeem miles for flights on US carriers.

UA, for example, is lousy with seat assignments for partner airline Star Alliance Gold elites. And many of the other Star Alliance airlines' programs have high fees applicable to award tickets on most/many of their partner airlines, while UA charges no YQ charges for award travel.

AA and DL's multi-tier/dynamic award pricing -- a move UA is going to be making too -- undercuts the ability to use partner airline program's miles for redemption on US airlines' flights. And while AA and DL are hooked to charging YQ charges for award travel, AA and DL tend to be less hooked to charging such high fees for award travel than a variety of their international airline program partners.