Neuroticism in the context of personality is not necessarily the same as having a mental illness or being "neurotic." There is some crossover between the behavior of someone we would describe as "neurotic" and someone with "high neuroticism" on the big five, but as a matter of personality, high neuroticism simply means that you worry more and tend to see things in a more negative, realistic way.

This carries obvious advantages in certain contexts as long as it is tempered and doesn't get out of hand. It has lots of disadvantages too, especially in the job market, but all personality traits have a downside if you score too highly. If you are too open, you can fail to weigh and measure risks properly. If you are too agreeable, people will walk all over you. If you are too extroverted, it can create dependency issues and alienation. If you are too conscientious, you can end up so consumed by the minutiae of life that you miss important, big picture ideas.