Our Marketo instance is synced with Salesforce. We have a couple of Custom Objects in Marketo that are linked to Company, where the Link Field is "SFDC Account ID", which is a custom field on the Salesforce Account Object that is identical with "Account ID".
Initially we were happy with the setup and it did what it was supposed to do: Show all Custom Object records on all persons in Marketo with SFDC Type = Contact with a specific "SFDC Account ID" synced down from the Salesforce Account.
But over time we saw more and more false results, namely contacts that didn't have specific COs associated with them. The CO existed in Marketo (invisible through the UI, but visible through API), yet it didn't associate with any contact. The reason was, so said Marketo support, that "SFDC Account ID" was not considered to be unique by Marketo, as it didn't have a 1-to-1 relationship with the internal Marketo CompanyID.
So Marketo Support said we should use "Company ID" as the link field, which is also supported by documentation:
Problem now: That field is not available in Salesforce, therefore not for our integration partner, so it cannot be part of the API call that creates the object. Has anybody ever successfully done that? Again: Our Marketo and Salesforce are natively synced, and as far as I know that makes a huge difference to the project, as the Company API in Marketo is blocked.