True but competent and ambitious generals in medieval societies don't mix well, if the troops are loyal to the general and not the Nation you're always one propaganda campaign away from an armed rebellion.
But in defense of appointing him, Aura is if i remember well the first female ruler they ever had she either has to bring untold prosperity to the kingdom or navigate the political game very well to keep her place, she can't afford to piss as much of the nobility as a male ruler can afford, after all we're one moderately sized mistake away from the traditional/conservatives families with blood ties to the royals to try and usurp her and install some male ruler in her place.
It's not true that they don't mix well. Autocratic societies, which include monarchies, have always rely on ambitious and capable generals. That very quality that makes soldiers loyal to a general is what keeps them loyal to the nation and the crown.
The key point is giving said generals, and other influential figures, enough to feed their ambition. That means political support, money, lands, and so on. This is quite literally the basis of the feudal system which yokes nobles to the crown using lands and assets. The nobles provide their own support in turn in various ways, but predominantly in military assets. The vast majority of soldiers owe their loyalty to feudal lords or marshals.
When a nation evolves its political system enough, the military begins to become centralized, and marshals rather than feudal lords take command of armies. They can do this because in a much more evolved society, which Capua is becoming, the legitimacy of the rule by the sovereign is supported by far more than military power. An armed rebellion is pointless when the judiciary, executive, legislative and civil institutions, particularly the latter, all refuse to support it. This is not something a mere single propaganda campaign can overturn, even for an unpopular monarch. In this sense Capua is similar to the fairly stable and institutions based monarchies of the Byzantine Empire or the various Chinese dynastic empires.
People often cite historical examples like the French Revolution as to how unstable monarchies can be once people get it into their minds that change is needed. In actual fact, the French Revolution was the result of slow erosion of the power of the French Crown due to serious economic, social and political factors, possibly over 60~70 years. Hardly a single propaganda campaign.
The fact that Napoleon later instigated his own coup...this time almost entirely military in nature...is further evidence of all the above. The newly formed government in the immediate aftermath of the Revolution was factitious and had hardly established its own legitimacy, with little established civil or political support. This, not the previous primary French Revolution, is the prime example of when a simple charismatic military commander is able to take over.
In this case, they're giving Puyol the position of Marshal to feed his ambition in the hope it's enough to sate it. Even letting him go ahead with his recruitment plan is part of that, headache as it is, to give him a sense of power, lest he seek others who can give it to him. They already implied it was partially to feed his ego after all. That's power he shares in part with those who support him, who also are kept happy, and so on. Obviously his ambitions can't be ignored, which is why they're so cautious. But this man becoming too powerful by himself is ultimately not as much of a threat as two steps from a coup. Else never mind not appoiting him, the position of Marshal wouldn't even exist.
