Google's PMax channel report, GA4's AI Assistant channel and Klaviyo delivery look like scorecards. They are mostly diagnostics. Here is what each can decide, what still needs an experiment, and what the public evidence cannot support yet.
Before MER goes in the board pack, marketing and finance need a written inclusion list for the denominator. The ratio is only comparable if that spend policy is fixed, restated when it changes, and read next to contribution profit.