The following contains spoilers for the Bleach manga Volumes 55 to 74, by Tite Kubo.
Tite Kubo’s Bleach manga has a tendency to foreshadow the power of characters, making claims of overwhelming strength that the protagonists simply have no hope of withstanding. Ichigo Kurosaki was warned of the might of the Gotei 13 Captains as he invaded the Soul Society, Toshiro Hitsugaya explained the power of Vasto Lorde Hollows and the Arrancar Fracción Shawlong Koufang taunted with the unimaginable power of the Espada. Whilst the above groups had their power stated, even if it did not always come true, there was one group in particular who received a brief mention, and the belief of their power came through implication.
In Chapter 108 of the "Turn Back the Pendulum" mini-arc, Shunsui Kyoraku first mentioned the mysterious faction known as the Zero Division. The dialogue was brief and came with little explanation, yet the seed had been planted. What group had the former captain, Kirio Hikifune, been promoted to? What was their purpose? Where was this Zero Division located? And the most burning question of all: how strong were they? When considering the concept of a promotion from the most powerful rank of Soul Reaper in Soul Society, it would imply a higher, greater power than even the Gotei 13. In reality, the hype was misplaced, and the disappointment of the Zero Division had been set in stone the moment Shunsui mentioned them.
The unfortunate truth of the Zero Division is that they were revealed as the manga was coming into its final stages. Ichigo and the popular protagonists were always going to be the heroes who stopped the invasion of the Sternritter, setting up the likelihood that the Zero Division members were going to tools to hype up the antagonists instead of making a great impact in the Thousand-Year Blood War. The overabundance of characters taking part in the conflict left little room for new, untested and relatively unimportant characters to take the spotlight. They served a purpose similar to that of the Fullbringers in the previous arc -- to train Ichigo and prepare him for his return to the fray on a new level of power.
However, it was not only the technical details of the narrative that led them to their rather dismissive defeat at the hands of the Schutzstaffel -- it was the reality of their position as inactive guards. Out of the five Royal Guards, only two portrayed a significant level of strength in comparison with a regular Gotei 13 Captain -- Ichibe Hyosube and Oetsu Nimaiya -- which made sense considering their place in the Zero Division. Oetsu was the father of the Zanpakuto, and his trade made him exceedingly familiar with the art of swordplay, which he performed expertly against the Schutzstaffel. Ichibe, as the leader of the Zero Division, was blessed with an ability with his sword, Ichimonji, that questioned the very fabric of reality.
To join the Zero Division, it was never stated that pure power was the route to a position; instead, it was a creation that benefited the Soul Society. Tenjiro Kirinji designed the Healing Hot springs, Kirio invented the Mod Soul and it is theorized that Senjumaru Shutara was the creator of the Shihakusho. These creations allowed them a place in the Royal Palace, and from witnessing their portrayal, it is clear a level of hubris came with such a station. They were, even if creative, still Captain Class Soul Reapers -- the same as the Gotei 13 -- who were handily defeated multiple times by the Sternritter during the first invasion and even some captains during the second.
The disappointing crushing of the Zero Division could be seen as a lack of time within the manga to truly showcase their power. This would make sense due to the same issue occurring with other characters in the final arc. However, when considering the facts of the story, the characteristics of the Royal Guards and their duty, which in reality meant little action for countless years, it makes more sense that they were perched in their palace enjoying a more luxurious life without considering the possibility that they would ever have been invaded. They were under-prepared and arrogant, and they suffered because of it.