Thanks for sharing your thoughts! That hasn't been my experience, target CPA was absolutely the driver of both volume and profitability for that business. You have to know how to work the algorithm and feed it the correct data. I was doing thousands of conversions a day so we had scale in our favor. If you're not careful tCPA can spend a lot of money for sure. YMMV though.
This is a basic application of the mean value theorem. Moreover, assuming 100% incrementality is what leads marketers to do silly throw money away things like Mailgun bidding on "mailgun" even though all the links on the first page of results are already to pages Mailgun controls.
Actually, that's not always that silly - if you have a structured campaign that uses ads to send people searching for specific things to the parts of your funnel you have optimized for them you can increase conversions and have a good ROAS even over the same keyword performance organically. It can totally be worth it to do.
