Although I agree that the dataset this paper uses is a bit limited, the main challenge that is underlying is the resolution of the images. I think that due to the low resolution of the objects within the image and given the specific application that they are addressinng, the common approaches (wavelets, etc.) don't necessarily perform their best. I'm not too sure how well other approaches that use similar size subwindows might perform in this dataset but, I'd guess that they would suffer some decrease in performance as well. Since not all the features observed in a subwindow of a high res image is captured within a low res image of equal size. Similarly, I also think that applying this approach on higher resolution images might produce worse results (or a result similar to a pure brute force search algorithm) than the common approaches.