I have had some troubles with one integration. It could be solved by doing a full re-sync which is not free. Every time I had to do it, I just sent them an email to tell them that I would do it and they did not charge for it.
Stitch, or Stitch Data, now from Talend (acquired in late 2018) is an ETL tool for developers; the company was spun off from RJMetrics after that company's acquisition by Magento. Talend describes Stitch as a cloud-first, open source platform for rapidly moving data. It is available on a Free plan, and also a Standard and Enterprise plan which include more advanced features (e.g. an account manager, multiple data destinations, HIPAA compliance, advanced scheduling).