As long as it was secure and no gremlins could get your nsec.
The problem is that your nsec is not just used for login. All events need your signature (posting, settings, ..) and every DM needs the key for decryption. You would need to hold up that card very often if your key should stay on the card. Our best option is a WiFi connected signing device that we can trust more than the phone OS. This would be a hardware nsec bunker.