There is an official and unofficial exchange rate in Burundi. If you exchange dollars at the official rate, you are getting ripped off as the black market rate is at least double or triple.
My suggestion is to bring dollars and contact a local that can connect you with a black market exchange. There are places you can go for this or people that can come to you. Use the AirBNB accommodation that I recommend because the host can help you exchange money.
Do NOT use credit cards here as you will get the much worse official rate.