Performance Max is Google’s automated campaign type: one campaign, all inventory, minimal controls. It can genuinely find volume that manual campaigns miss. It can also produce a spectacular-looking report by harvesting traffic you would have converted for free.
The brand problem
Left unrestricted, PMax will serve on searches for your own brand name. Those searches convert at a high rate and cheaply, so they flatter the campaign. If PMax launched and immediately reported excellent performance, check how much of it is branded before celebrating. Brand exclusions are the first thing I add to any PMax campaign.
The remarketing problem
PMax also reaches people who have already visited your site, and those conversions were often going to happen anyway. This is not fraud, it is attribution: the campaign gets credit for demand it did not create. Judging PMax on incremental new customers rather than total conversions changes the picture considerably.
Where it genuinely earns its place
With a strong product feed in retail, where feed quality does most of the work. In accounts with real conversion volume, where the automation has enough signal to learn from. And as an expansion layer once search demand is already being captured properly by Search and Shopping.
How to keep control
Segment asset groups by theme or margin rather than dumping everything into one. Exclude brand terms and existing customers. Use audience signals as direction rather than targeting, and check whether they helped. Mine the placement and search category reports for the limited visibility Google does provide, and exclude aggressively.
How to judge it fairly
Compare against Search and Shopping with the same attribution and the same time window, after brand exclusions are in place. Look at new customer acquisition, not blended conversions. If PMax still wins, it deserves the budget. Frequently it does, and just as frequently the honest comparison changes the allocation. Either answer is useful, which is more than you can say for running it unexamined inside a managed account.