Our efforts have sterilized and eartipped more than 7,000 feral cats. One thing that is very important is that all groups working with feral cats agree on a universal method of identifying sterilized animals. Feral cats may interact with a variety of caretakers, vets, and animal control people during their lives. If every group has a different method of identifying the animals, there is no way to interpret what the marks mean. It is very traumatic for a feral cat to be retrapped and transported because of unclear markings.
Tattoos are frequently unreadable in feral cats unless they are anesthetized.
Eartipping cannot be considered inhumane. It is performed under anesthesia at the same time as a major surgery and is certainly less traumatic than a spay or neuter. The worst possible thing is to perform unnecessary surgery on a cat that is already spayed because it was not marked with a universally recognized symbol, and this I have done. However, many people forget that tattoo equipment should be autoclaved between each cat to prevent the spread of FeLV, FIV, and viruses we don't know about yet. Cold sterilization will not work when there is blood, tattoo ink, and hard to clean crevices on equipment.