One of the main reasons for the anime groups obsessing about the comparisons is that it's a lot easier to measure final quality on anime, with it's clearly-delineated artwork, instant transitions, and (comparatively) simple backgrounds.

I would also have guessed that many approaches for video/image compression like assuming lots of nice smooth gradients without hard edges, and so flat shaded but detailed drawn things might fare badly.

Hard edges are difficult for the DCT/DST used in most video codecs. A wavelet-based codec would most likely be better suited.

However, all modern codecs that I know of have directional intra prediction, which should be pretty effective in anime. Basically, what it does is that the decoder is able to follow lines with a variety of angles.

There are other things that help compression in anime: static frames, large flat areas, lack of film grain in computer drawn animation.