You can mitigate your concerns about population and crime rates by investing in better neighborhoods. See South Wedge and some areas of the 19th Ward. I haven't looked much into it, but I've heard the Merchants/Culver area is picking up steam.
I'm guessing you're looking in Rochester neighborhoods where a $50K duplex or $25K single is the common offering. As an out of state (and out of country) investor, I would not recommend this. If you are looking to invest in the low-end markets, expect low-end tenants. Which means non-payment of rent, frequent damages and repairs, crime, low and declining rents, and headache after headache. Your properties will get beat on and it will show. Good PM companies to manage these types of properties are tough to find.
The prices of these properties are so low because they reflect the condition of the area, type of tenants, and the quality of surrounding housing. I would only consider these if you are local, have experience (with rehabbing and the active management of tenants), and have cajones of steel. Emphasis on the latter.