Not just back in the day, I have at least a couple of customers of which I know they are losing money because of billing issues.
Why? Because most accounting software is horrible and have minimal to no third party integration options. Many of them don't have an API, so the only option you're left with is manually (!) importing some XML structure every day. The logic is usually very limited so stuff like record mutations are often silently ignored during the import.
The accounting software world is evil. Deliberately engineered vendor lock-in, very limited third party integrations, ridiculous charges for 'custom' implementations, etc. I once build a migration tool for a well known accounting/ERP suite, and my partnership with them was immediately terminated due to 'breach of terms'.