dont let people scare you. you will find people who are scared to leave their front door, scared to get out of bed!
i spent time in all of those countries and there is not a week goes by that I dont think about going back. Becoming infected with an unforgetable love for that piece of the world is the risk. Sure there are unscrupulous people anywhere, but the vast majority of people are wonderful and generous and amazing. You are in for a treat!
As for dangers, bus travel can be a bit hair-raising, but probably if not a lot more risk than commuting daily in traffic in major cities in america. In Nairobi you want to have your wits about you and be smart, dont wear watches or jewelery or flash around fancy stuff. When I was in Dar 6 years ago i loved it and felt quite safe. I am a woman who travels alone- went by cab out to hear bands both there and in Nairobi in the eves a lot. Loved the place! You are lucky to get a job there!