Union City is just that, a city, so as a woman you should always be alert when alone, especially at night. That would be the case in any urban area. For more specifics on the crime rate, you might contact the local police department and they can give you some specifics. You should spend some walking around, too, to get a feel for the neighborhood, both during the day and at night.
Recently Union City has become a very popular alternative to pricier nearby neighborhoods like Hoboken. There is good transportation into the city by bus or you can take the bus to the PATH station in Hoboken. As it becomes more and more gentrified, along with the demographics of the area changing, the investment potential also improved. Just look around and you can see all the new, high-end residential construction taking place today. So, in my opinion, in the long term (i.e., 5+ years) it is a great investment, especially if you plan to live there while your asset - your home - increases in value. I specialize in Hudson County so feel free to contact me if you have any questions.