We all know that Bitcoin is also popular to criminals because they can use it without exposing their identity. It will be more convenient to them to use Bitcoin so that they can hide their identity. Do you think that terrorist groups like ISIS use it for their transactions?
There's a possibility that ISIS or other terrorist organizations are using bitcoin. Bitcoin is the best, convenient and easiest way to transfer funds without even getting attention from the government, military, or police. I think most illegal transactions can use bitcoin transaction because it is safer than sending money in bulk or delivering it.
It's safe but they can be tracked too. They may use BTC but it's probably not their favourite way to pay or send money. Physical cash remains the safest option to me