On my last flight I had a tight connection at ORD. The flight was delayed due to “late-arriving bags.” My bag showed arriving in ORD, but never showed as loaded onto my connecting flight. Yet, my AirTag showed the movement of the bag, and showed it “with” me just before we pushed back. It went “dark” until a few minutes after we pulled up at the gate at AVP, after which the AirTag showed the bag at AVP. My guess is that it slipped through the conveyor without getting scanned onto the flight.
So, UA’s app is great for piece of mind when all goes well, or when the muck-up is obvious. But, it is not fool-proof. AirTags are excellent companion pieces to the UA baggage tracker.