A plane acts like a
Faraday cage, and can block many signals. (There aren't any windows in the cargo hold) And I'm sure that some planes block signals better than others.
But, so far they have kind of worked better than the baggage tracking on Delta. I was on a flight earlier in the year and the return flight didn't get me a notice that Delta loaded my bag, yet the AirTag was 'with me'. Outbound, the AirTags showed they were in the main baggage sort at ATL, which gave me some angst, but after opening the Delta app, saw they were loaded (I then turned on notifications in the Delta app🤦🏻♂️). It's not perfect technology because reality, but it can be a powerful tool to keep your stuff with you. Just not likely in planes, trains, buses. On reflection, the experience was 50/50. Delta won one round, and Apple won the other.
I'm sure they will really help in the baggage claim. A decade ago, or more, someone grabbed my main suitcase and headed for the door. If not for the eagle eyes of my wife, they might have gotten away with it. They also claimed they 'mistook your suitcase for theirs' and yet went out with one that didn't look anything like mine. Hmm... Dementia, or deliberate.