On that note, expect to lose money initially while optimizing your website & waiting for enough CPA data to accrue. I give it at least a month and $5-10k spend before seriously considering killing the campaign (if starting from scratch).
Very different from taking something already successful but mismanaged - this can be a lot easier to ramp off if they're making big mistakes (like not actually mobile optimized, not CPA bidding, not actually actively optimizing the website's funnel/landing page).
This is spot on! When I first transitioned those campaigns from eCPC to tCPA there was a big learning period. As the campaigns scaled and collected more data the actual CPA started aligning closely with the target CPA, but it took some time. The more conversions you do the faster that alignment will happen. After 2+ years on tCPA (and many behind the scenes improvements to the Algo by Google), I could get that delta to match within 3-4 days of making a change to the target CPA. That was with thousands of conversions a day though, so scale absolutely helps.
Very different from taking something already successful but mismanaged - this can be a lot easier to ramp off if they're making big mistakes (like not actually mobile optimized, not CPA bidding, not actually actively optimizing the website's funnel/landing page).