If the Lyft app is installed on your iPhone, you can ask Phone Siri to order you a car. But you can’t ask Mac Siri to do the same, because she doesn’t know what Lyft is. Compare and contrast this with the SDKs for Alexa and the Google Assistant – they each run third-party software server-side, such that installing the Lyft Alexa “skill” once gives Alexa the ability to summon a ride regardless of if you’re talking to her on an Echo in your bedroom, a different Echo in your living room, or via the Alexa app on your phone. Many Siris · Bryan Irace

This is ultimately where the Apple privacy stance falls down. With on device processing Apple are unable (or unwilling) to make abilities available everywhere.

They could in theory sync encrypted information to other devices. Or allow for account information to be stored in iCloud, however this again breaks Apples privacy ethics.

There may soon become a time when Apple needs to back down from such a view point and give users the ability to opt in. Until then don’t bank on Siri being able to do everything everywhere.