Which system should own the customer data?

The question is framed as if one system has to win. In practice, they each own different parts.

Break it down by field

The CRM usually owns contacts, relationships, and communication. The accounting system owns the organization number, billing address, and payment terms. Both are correct, and the boundary must be documented.

One writes, others read

For each field: one system writes, the rest read. When two systems can both write, discrepancies arise that no one notices until someone compares them.

Practical approach

Create a simple table listing the fields that exist in both places, and assign an owner to each row. It takes an hour and resolves a debate that would otherwise drag on for years.