As a YNAB user, I would be concerned that the mobile app cloud sync would stop working in the future and not get updated.
Agreed, at some point SDKs will break it.
The only cloud sync in YNAB 4 is the one via Dropbox, where it shares the budget file with the desktop app. So the only issue is the DB API being updated.
Yes, would love to build something like `hledger add` for mobile.