I don't think Nightmare's statements are vague. With the rough translations we're given you see them talk about Nightmare's power which is referred to as "infinite" and say something to the effect of "if Nightmare keeps releasing it's infinite power, the stability of the demon world will shake/waver/fluctuate etc." or this automatically means it's refering to power and nothing hirarchal since it's in the context of Nightmare's literal power being released over and over being the cause of the shaking.
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Nightmare
It is not known if there is an inorganic existence to follow instructions from the emperor just mechanically, and it is possible to shake even the stability of the demon world if it keeps releasing the power which can be said that it is infinite, and it is this that it was said that it is a failure work above. 'The impatience devil will be arranged with nightmare in the restraint pattern (restraint tool to suppress runaway) set'.
"Nightmare.
This is the reason why it is said to be an inorganic existence (unknown whether there is a will) that performs merely a mechanical command of the demon emperor, and the stability of the Makai might be wavering if it keeps releasing the power which can be said to be infinite, and it was said that it was a failure in the above. 'The impatient demon emperor will be placed in a restraint pattern (restraint to suppress the runaway) set with nightmare."
"Nightmare.
An inorganic entity that mechanically commands the Demon Empire (it is unknown if there is an intention), and if you continue to release its infinite power, even the stability of the Makai may fluctuate. The imperial Demon Empire will be placed in a restraint pattern (restraint tool to prevent runaway) set together with Nightmare, he has the power to destroy the demon world"
Even if this quote is not accepted the other one still points to pure destruction though ,once again, this is based off the translations from this thread.According to this translation, Nightmare was created with the intention of invading the Human World and purely destruction but Mundus being a big dumb, created a monster that was entirely too strong and difficult to control so it didn't work as intended (which is a running theme with all of the Nightmares created by Mundus) and was a threat to the demon world itself enough that it could destroy the demon world according to the description and Dante's own words being "Honestly it would be nice if the demon world runs away and the demon world itself is destroyed." This is obviously not refering to anything hirearchal and it wouldn't make sense for Dante to joke about letting Nightmare go to the demon world and destroy the hirearchal structure verses Dante letting Nightmare go to the demon world and literally destroy it.
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"Nightmare It is a living weapon and a senior devil created by the Emperor with the intention of invading the human world. However, because it was made for the purpose of destruction only, it was difficult to control and even the magic world could be destroyed, and Mundus who felt impatient with this decided to use it only within the boundary of the crest that would be a restraining garment. In the shape of a slime with a lot of bones floating, you take your opponent into your body and fly to the nightmare space. In order to give damage, it is necessary to activate the surrounding barrier device and make the main body appear. However, even if the main body appears, the defense instinct increases, handling weapon attacks such as missiles and beams, and transforming parts of the body into spears and boomerangs to attack. In addition, Dante commented in the above strategy book, "Honestly it would be nice if the demon world runs away and the demon world itself is destroyed."
Once again this is all based on the translations given in this thread, it would be better to find more translators and get a better idea about what is being stated here and the context but, based on what we have now, these quotes can't be taken as hirearchal.Also, the "Its power can lay waste even to the Demon World" quote is vague enough that it can also refer to actual destruction aswell which seems to be the case as of right now.