The "huge" disadvantage of the physical Card is that the card number ISNT printed on the card. Nor is the expiration date or cv code.
While this may possibly enhance security, it requires you to either memorize all that information or ALWAYS have your iPhone with you when needing to make a credit card purchase that requires entering the cc info.
The beauty of a plastic normal traditional credit card is that you don't need to carry anything else with you. All you need is the card. With the Card, that's not guaranteed.
If someone were to only get one credit card, the Card (IMO) would be a poor choice. It seems more like a major liability to me, locking a credit card to your iPhone .
Apple is spending billions of dollars trying to diversify away from being as dependent on iPhone sales as they've been by starting all these new services. They're attempting to do that at OUR expense by trying to monopolize everything in our digital life.
If people were smart, they'd follow Apple's example and diversify AWAY from Apple services. Don't be their patsy.