Although having a very low unemployment rate is better than a high unemployment rate, it doesn't help paint the whole picture of the American workforce. The other metric that ought to be mentioned along with unemployment is the labor participation rate, which has been trending worse for a while now [1]. Considering the two together, the economy isn't as rosy as it looks. [2]
For example, if I were to be chronically unemployed and quit looking for jobs altogether, I would actually help the unemployment rate look better since I'm no longer part of that metric.
Part of the problem can be attributed to automation in jobs and job duties, thus requiring a smaller labor force. For those who were laid off by automation in manufacturing or other sectors, many would likely have a hard time finding a job in their established career. Expecting most of such workers to get a job an an on demand skill like maintaining machinery or software development is a herculean task. Most likely they are the ones who fall prey to the poor labor participation metric.
[1] https://www.investopedia.com/ask/answers/061515/what-key-dif... [2] http://www.startribune.com/even-with-unemployment-at-50-year...