The Kralhi weren’t failures, they were just given the wrong job
I’ve been thinking about Nuparu’s first attempt at robotic law enforcement for Metru Nui and how they honestly could’ve outperformed their replacements if they had just found their niche.
If we look at their history and abilities, yes, it’s clear why they didn’t succeed at law enforcement:
But honestly, I think their energy draining abilities would run rings around the Vahki in terms of dealing with dangerous Rahi. The stun staffs of the Vahki likely have a similar limitation to mind control powers within the MU in that you need to have a complex enough mind for them to really take effect. And even if that wasn’t the case, some stun staffs powers would be actively useless against Rahi (e.g. the spying powers of the Rorzakh) or could make a rampaging Rahi situation actively worse (e.g. the confusion powers of the Keerakh). Instead, the Vahki would most likely have to rely on their Kanoka to deal with Rahi incidents, slowly wearing it down through various Kanoka powers, powers which seem to wear off relatively quickly for the offensive types.
But an energy drain? Something that could potentially make the Kralhi stronger at the same time as making the Rahi weaker? Something that makes the Rahi docile and weak enough to handle on the order of days, allowing easy transfer out of the city or into the Archives? That seems to me like a much quicker and more efficient method of dealing with a Rahi problem than anything the Vahki could do -even the specialist types like the Kraahu.
Honestly, it wouldn’t even surprise me if Rahi containment was the original design pitch for the Kralhi, with the pivot to law enforcement being an order from Turaga Dume.
If that was the case, then it wasn’t their fault, or Nuparu’s.
They didn’t fail because of design, they failed because of poor project management.