There have been some interesting stories especially the ones with Hive Fleet Gorgon who’s extremely rapid adaptability is astounding even by Tyranids standards. In the end the Tau won out due to being more flexible with faster adaptability against what is know as the most extreme rapid adaptability Hive Fleet.
It wasn't quite as one sided as you make it sound. Gorgon was actually doing fairly well when the war was strictly between itself and the Tau, but ran into trouble with a third party (a lost AM
regiment) joined the fray. The problem was it was so optimizing its forces to deal with one opponent that it was drastically compromising its efficiency against the other one. As an example, one of the earlier waves of gaunts incorporated a mucus secretion on their exoskeletons that was specifically formulated to absorb and disperse pulse weaponry, but the extra coating had no effect against solid-shot projectiles and reduced the overall speed of the organisms so modified, for a net negative against non-Tau.
On the original topic, I think I like the look of the new Shadowsun overall. The added armatures are a bit odd but I think part of it is also the color they used making them clash with the armor. I imagine having them the same color as the plating would make them less conspicuous. From a mechanics standpoint, I think they can be justified as extra stabilizers for her new weapons. In the original Dawn of War fusion weapons had quite a nasty recoil to them, and given her new ones are presumably prototypes intended for a larger suit, it could well be that additional limbs were needed to keep the weapon's kick manageable.