We don't have access to the internals of these system obviously, but we've worked very very closely with some of our customers to make sure nothing weird is hap

We don't have access to the internals of these system obviously, but we've worked very very closely with some of our customers to make sure nothing weird is happening after switching. It previously required a lot of fine-tuning, now it's usually quite smooth. For the FB Pixel, we're doing both :) We do use the Conversion API (with the first-party cookie), but we sometimes do reconstruct the GET request on our backend and pass the final URL to the browser to execute. Most tools are not as complicated though.