One of the things that Cicero himself recognizes about audacity is that it is courage-like. The recognition that not all vices are diametrical opposites of virtues goes back to Aristotle. Aristotle, you will remember, held that every virtue is a proportionate mean between two vices, one of excess and one of defect. However, he does not hold that the virtue is in the exact middle of these two vices. Of the two, one vice may well be much closer to the virtue than the other. If the virtue is a restraining virtue, requiring self-control, then a vicious excess of passion will be more opposed to it than a vicious deficiency of passion. On the other hand, if the virtue is one that requires pushing ourselves to continue, a vicious deficiency of passion will be more opposed to it than a vicious excess. So it is here: audacity is much closer to courage than the opposing vice, which goes by different names, but which we can here call timidity. It is nonetheless a vice, and its similarity to courage to some extent makes it even more dangerous than timidity, because the audacious can convince themselves easily that they are virtuously courageous, whereas the timid cannot.
We find exactly this view given by Aquinas (ST 2-2.126): audacity is a vice opposed to fortitude as its vice of excess. He interestingly identifies three sources of audacity as a vice. The first is that there is a close connection between fear and love. Who loves something, like his own life, will naturally fear to lose it, and therefore one cause of audacious behavior is a deficiency of love for whatever is on the line. We don't deliberately risk the things that are truly important, if we can help it. The second source of audacity is pride. One may love one's life and rashly risk it out of a sense of invulnerability. One way, in other words, to cultivate the vice of audacity is not to appreciate the fact that what is good can be lost, on the mistaken view that you can handle whatever might happen. The third source of the vice is culpable stupidity, the simple failure to think through the consequences of what you are doing. All of these are found in Aristotle, but in somewhat scattered way; Aquinas is reorganizing and simplifying Aristotle's approach, as well as generalizing it, since Aristotle tends to talk about audacity in the midst of battle, whereas Aquinas carefully avoids confining his discussion to any particular kind of situation.
In all of these audacity opposes the good life; as Aquinas says, commenting on Aristotle (In Nic. Eth. III, lect. 14, sect 533):
The brave man is praised because he does not fear. But there are certain things which we ought to fear in order to live a good life. It is good to fear these things inasmuch as fear is not only necessary for the preservation of respectability, but even fear itself is something honorable. There is a kind of disgrace attached to the person who does not fear evils of this sort.
The insistence on audacity being a vice, in other words, is tied up with an insistence that some fear, caution, hesitation, is honorable. Fear is not different from any other passion in this respect: it has its honorable forms and its dishonorable forms. The fact that some fear is dishonorable, however, does not mean that all fear should be avoided.