Instant army with nanites

This question's main focus is the building aspect of the nanites.

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Rulez:

• No pseudo-science.
• No handwavium.
• The backstory is not (that) important.
• Lame puns ahead, proceed with caution.

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Properties of nanites, in general:

Nanites can be considered organic from many perspectives, and are mainly composed of carbon. They utilize many tricks from Earth's nature and powered with induction charge. their size is somewhere around tardigrade's, and are capable of withstanding the same amounts of various environmental hazards as a tardigrade. Most nanites are specialized to the execution of one specific task, but all of them is commanded with a quantum computer.

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How they build:

• A mass extractor mines some ores.
• The ore gets broken down with the help of plasma torches.
• Nanobots placed into this newly made substance.
• The nanobots absorb all the useful stuff from the substance and swell up.
• Then the nanobots cast into a casing (The casing is a hollow, 3d-printed version of the structure we want to make.) of the where they rearrange themselves, and start to produce a "connective tissue" between each other and shrunk down.
• After that, the nanobots will remain in the built structure deactivated, and if needed, reawaken and repair it.

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What they build:

Tanks, planes, Spaceships, the Deaf Star, pretty much anything that the commander has a plan of.

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Questions:

• What are the things, that could make them unrealistic?
• How to improve their building speed?(for the sake of simplicity let's just assume that they don't use any)
• Improvements in their techniques in general.

Feel free to focus on one question at a time, you don't need to have all the answers, one is more than enough. Thanks!

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Backstory:

The Obligatory Instant Robot Army was used, in The First Galactic PWNage Campaign with unprecedented success, crushing the Ork, Dark Eldar, Chaos and Tyrranid armies/forces altogether. This can be accounted to the fact, that the O.I.R.A used common elements such as carbon and dead foes and reinforced them with pure nano-level engineering.These key factors, in the end, created an ever growing army, that fed on your casualties and tears.