If the marketer is an employee or a consultant, is it in their interest to show that the ad-spend they are controlling is high ROI, or low ROI.

Maybe this is a cynical take, but, if they get to the bottom of things, and show their boss/client that the ad-spend is not returning so much, it seems it would portend bad things for the marketer.

I really don't know, and it seems like a very hard problem.

Maybe this is the time for that Upton Sinclair quote: "It is difficult to get a man to understand something, when his salary depends upon his not understanding it,"

That's not how ad performance is measured. Your ROI is based on end conversions. You need to know if your leads are _good_, not just plentiful. A company that doesn't do this at the start will figure it out pretty quickly.

Ideally that would be the case that all ad-spend can be tracked through end conversions, and for many businesses it can be. But for much of the corporate world, you don't know where your end conversions come from. (Think Nike shoes, etc) It's estimated about that 35% to 45% of all digital ad spend goes to brand awareness, video reach, and other upper-funnel methods where direct conversion tracking isn't possible.