I just discovered Apple note which says that Faces are note synced across Macs.
That is a bit confusing as smart albums are synced, and smart albums can contain face-related conditions.
In any case, I was extremely disappointed to read this and it actually stopped my plan to buy storage and move to iCloud Photo Library as I invested infinite time in tagging faces in my very large library.
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iCloud Photo Library allows you to access your photos and videos on multiple Mac computers, as well as your iPhone, iPad, iPod touch, and iCloud.com. Just sign in to iCloud with the same Apple ID on all your devices. You can turn on iCloud Photo Library in Photos > Preferences, or from the iCloud pane in System Preferences.
After you sign in, these items appear in the Photos app on all of your Macs:
- All original photos and videos
- All folders and albums
- Smart Albums
- Keywords
- Searchable keywords based on Faces tiles
- Key photo selections
These items are available only on the Mac where you created them:
- Books, cards, and calendars
- Slideshows
- Keyword shortcuts
- Unused keywords
- Last imported album (this album contains photos you most recently imported on a specific Mac)
- Faces tiles and Faces data
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