AirTags bring the advantage of locating your pet quickly and precisely with the map feature. The technology works based on iPhones being near the pet to capture the signal. If your pet is in a remote area there may not be a signal until reaching a more populated area. There are approximately 135 million iPhone users in the U.S. so the chances are high of receiving a signal. Notifications can be setup to let you know as soon as a signal is received.
Microchips are a great idea and should still be used. They are implanted and can't be lost compared to a dog's collar that can come off. To use the microchip, it requires the lost pet be caught and taken for scanning which delays identification, however. Microchips also require the owner to maintain current contact information so they can reach you when needed.
An AirTag is a good tool for finding a missing pet quickly but should not be the only approach. No single approach can guarantee success, so use several so you can locate your pet as quickly as possible and set yourself up for the greatest chance of success!