That is a great question. Some feature parity would be needed; I appreciate much of the Xero API, though some parts are incomplete. Built-in support for Stripe is great. Both have been beneficial for integrating with some business processes, notably reporting.

More flexible reporting would be a killer feature. They have standard reports, but they aren't flexible enough for a nonprofit, and the reports through the API are pretty lame. I have to pull 7 reports just to automate a proper balance sheet, because a) I don't want to export reports manually and b) the API only lets you filter by a single tracking category at a time.