Trying to plan ahead so any know if card payments are common at hostels in Burundi or cash is king?
I'd be very cautious about relying on card payments. The reality is that the financial infrastructure for card transactions is not widespread, particularly outside of the capital, Bujumbura. The vast majority of hostels are not equipped to take cards. The best plan is to withdraw a good amount of local currency, the Burundian franc, from a bank like KCB or Ecobank upon arrival and use that for your hostel payments and other daily expenses.
Don't even bother with cards. I stayed at a few hostels and small hotels in Burundi, and not a single one accepted a card. Even at the main banks, getting cash from an ATM can be a bit of a challenge with a foreign card. You will definitely need to bring US dollars to exchange or use a money transfer service if you need more money.
Cash is absolutely king here. I was just in Bujumbura a couple of months ago and tried to pay with my card at a guesthouse, and it was a complete no-go. The place was not set up for it. My advice is to assume that you will not be able to use your card for hotel. You'll need to go to a bank to get Burundian francs to pay for your stay.
You're going to need to have cash on hand. The vast majority of hostels and guesthouses in Burundi, even in the capital Bujumbura, do not accept credit card payments. The financial infrastructure for cards is not widespread, and most businesses, including hotel, operate on a cash-only basis. The local currency is the Burundian franc. You will have to withdraw cash from a bank like KCB or Ecobank to pay for your stay and other expenses.