The metrics are often broken. Think about how you measure it...
Ideally, you want to pay the ad platform for sending you a user who will buy your product. So you say something like:
"I'll pay you X for every user you send me who buys something".
Great, right?
Except now the ad can simply optimize for users who were GOING to buy your product anyway. So now you've spent money getting users who would have been there anyway.
What you really want is to pay for users "Who buy my product, but would NOT have bought it without the presence of the ad". And that's a much harder thing to measure.
Ideally, you want to pay the ad platform for sending you a user who will buy your product. So you say something like: "I'll pay you X for every user you send me who buys something".
Great, right?
Except now the ad can simply optimize for users who were GOING to buy your product anyway. So now you've spent money getting users who would have been there anyway.
What you really want is to pay for users "Who buy my product, but would NOT have bought it without the presence of the ad". And that's a much harder thing to measure.