Performance Max works. That is the confusing part. It works well enough, quickly enough, that it is easy to stop asking what it is spending on.
The trouble is that the reporting is deliberately thin. You cannot see search terms the way you can in a standard Search campaign, which means the query management that actually controls cost is happening somewhere you cannot audit.
When we run it
For accounts with clean product feeds and genuine breadth, it earns its place. For lead generation with a narrow service list, it usually spends into audiences that were never going to buy.