I say dangerous because they have a higher crime rate in contrast to cities like New York, Philadelphia or Houston for example.
This is an enormously thorny subject, but a very simplified answer is that Detroit and Baltimore both suffer from extreme de-industrialization, leading to poverty and thus to crime. New York and Houston are both in pretty good financial shape today; both were able to retain a key industry which wasn't vulnerable to foreign competition (Finance in NY, oil and gas in Houston) and supported the service economy through economic downturns.
That said, some cities made decisions that compounded the loss of industry, and interestingly, some had pre-existing conditions that exacerbated the societal effects of that period. David Simon, in his nonfiction book The Corner, mentions that Baltimore already had some of the highest rates of heroin use in the US well before the local economy tanked, although he doesn't really offer up an explanation as to why, and I've always wondered.