I would like to adopt a retired racing greyhound someday. They are crate trained for racing, and adopters sorta "switch them" to house-training. Obviously my future dog will have his own crate and have to stay in it now and then while getting acclimated.

Training in itself is not cruel, but it can be abused. Any training or care method can be abused either deliberately or simply out of not knowing how to do it properly.