Most of the weaknesses associated with any given personality type are a result of that type's dominant function overtaking the personality to the extent that the other functions become slaves to the dominant function. Although it is natural for every personality to be ruled by its dominant function, it becomes a problem when the supporting functions are not allowed to develop fully on their own because they are too busy "serving the master". In such cases, a personality can become quite imbalanced.
A situation in which the dominant function of a personality completely overshadows the other personality functions is analogous to a kingdom that is ruled by an overbearing king who requires absolute servitude. Imagine such a king sitting down to dinner in his castle. He keeps all of his servants running about to bring him dinner, and requires that they serve him fully (disregarding their own needs) until he is completed sated. His Foreign Minister, who is expected at an important affair at a neighboring kingdom, finds himself pouring ale. His Minister of Domestic Affairs, rather than addressing the issue of a failing economy, slices roast turkey. His staff grabs food for themselves here and there, but never get what they really need or want, and are consequently unsatisfied, malnourished, and underdeveloped. The issues that the staff should be taking care of are left undone, because they never finish their primary task of serving the king. The king's immediate needs are being met, and so he is tolerably happy, but he is an ineffective king. As far as he knows, everything and everybody exists simply to serve him. He has no concept of Success beyond his daily needs. Since he cannot see beyond his own needs, the entire kingdom suffers.
Likewise, a personality that has developed with a goal of serving the dominant function above all other considerations often results in a person who is imbalanced. In severe cases, the weaknesses associated with the given type are often quite apparent to others, and overshadow the individual's natural strengths. Such a drastic imbalance is not common, and may be the result of continuous and extreme stress. Most people will experience times in their lives during which they are stressed to the point of serious imbalance. People who experience this constantly have issues that need to be dealt with, and should seek help.
Much more commonly, we see individuals who exhibit both the strengths and weaknesses of their type. It is natural and healthy that each personality type is ruled by a dominant function, and that the other functions support the ruling function. We don't seek to change anyone's natural self, or to achieve a perfect balance amongst a personality's functions. By definition, a kingdom needs a king in order to exist, and a personality needs a dominant function. However, a kingdom with a well-developed and effective king (the dominant function), who has well-trained and educated advisors (the supporting functions), will thrive more than the kingdom ruled by a neglectful king who is supported by inexperienced advisors.
As we can see, Balance and Success are relative terms. They have different meaning for each of the sixteen personality types. One statement using these terms is true for all types: Balance is the key to Success.