In practice
Attribution answers who gets credit; mix modeling answers what actually worked. The most useful engagements treat the model as one input to an ongoing conversation, calibrated with experiments, checked against blended metrics such as MER, and refreshed as the business changes.
The most common mistake is expecting a model to deliver a single, certain answer. A good marketing mix model gives ranges and scenarios, and it earns trust by being validated against experiments and sense-checked against simpler blended measures.