I live in Flatbush now. between Church & Albermarle. I will say this: at night, you might see "drug activity" (dealing). But I also realized that if you don't know what drug activity looks like, you may not even notice it. Despite this, I do NOT feel unsafe. I see cops around in the evening and at night all the time. The area is heavily Carribean and most people mind there business, or they're reasonably friendly.
Be cautious of the buildings you rent in this area. My building is a mess and I plan to move soon. Try to talk to tenants if you can and see how they feel (if they have heavy mice, bedbug, roach infestations). A lot of these buildings have section 8 populations and are not well kept. My building is "transitioning", and the managers are used to being lazy and unresponsive because the government pays them directly anyway. Do your research.