They’re serious about that “unlimited” thing. I set a client of mine who does Estate Sales up with Google Photos, and she is constantly shooting photos, all day, every day. She currently has more than 19,000 up there, and can easily search for a “table” and be pretty sure of seeing every table she’s had in a sale.
With the original Pixel, it is truly unlimited, no sizing restrictions. There are caveats like the compression (downsizing) for other users, and they pared it back to the standard offering with the Pixel 2. But yes, original Pixel owners get unlimited no-strings-attached free photo storage.
The downside in any case is you don’t physically possess the data. As with any online storage service, there is no guarantee it will always be there. And if it ever disappears (for whatever reason), you have no recourse.