Hi Anthony,
Actually parent_geo_target is a dumbed-down version of ParentLocations:
1. It skips some hierarchy. Example: according to parent_geo_target the postal code SW4 (id: 9045894) is a child of England (id: 20339). But ParentLocations accurately includes the city London (id: 1006886).
2. In addition it can't replace ParentLocations as some locations actually have multiple parents. Example Kwinana Town Centre (id: 1000658) is actually a child of both Perth (id: 1000676) and Southern & Fremantle (id: 9060985). Which can be derived via the ParentLocations field and verified in the Google Ads UI.
To switch from the Adwords API I would need the equivalent of the ParentLocations field that returns all of the location's parents.
/Sebastian