Thought I'd update this post to help anyone else who might be considering integrating to Xero. I ended up purchasing two integration apps from the app store, thoroughly testing them and documenting the issues. After presenting the issues to the accountant and the client, and having a 3-way debate about the pros and cons from each perspective, we all agreed that the risk of integration was too great and the accounting is now being done on Odoo. Integration might be the right solution for a different client with different circumstances but it wasn't the right approach for this project. Some of the issues we experienced were:

* Manual journals to Xero would have been required for inventory adjustments for this manufacturing client
* Different field lengths in Odoo and Xero resulted in data exchange failure
* Error messages that were indecipherable to anyone except a developer (there were numerous throughout the testing period) - this would have caused a high support cost and inevitable dissatisfaction for the client
* Poor responsiveness from some of the app developers, even during pre-sales.
* Client was moving from a legacy desktop system so their internal accounts team would have had to learn and use both Odoo and Xero - high friction for users, especially those already anxious about the change.

As with any integration, I would recommend thoroughly testing your proposed solution. It took me a lot of time and effort to do these evaluations and present the findings to the other stakeholders but I feel it was worthwhile to avoid bigger issues down the line.