Custom audience
An audience built from the advertiser's own data: site visitors, a customer list, app activity or engagement with the brand's content.
What makes it wrong.
The mechanism, not the warning.
Audience membership expires. A thirty-day site visitor audience shrinks continuously unless traffic replenishes it, so a campaign that performed well at launch can be delivering to a fraction of the intended pool weeks later without any setting having changed.
In detail.
Custom audiences are the input to both retargeting and lookalike expansion. Their quality decides both: an audience built on all site visitors behaves very differently from one built on purchasers.
They carry personal data by construction, which is why the matching happens on hashed identifiers and why the platform reports a match rate rather than a list.