I did some integration work with QuickBooks online, and oh the horror! Simple things like reconciling transactions against a bank account couldn't be done via the supported API. There was basically no way to automate clicking on every single customer payment aside from scraping the website.

Bonus: the website screws up when you have thousands of transactions that need to be reconciled. I couldn't come up with a good test case as it seems to be some weird internal race condition.

In other words: I wholeheartedly welcome an open source project my business can build on.