Most popular criticisms I've heard is that Bitcoin was created by or for terrorists, or widely used by them or people for shady deal. Due to it being completely anonymous and untraceable. I doubt that it is fully untraceable, because despite you dont know address owner, but you can still follow all his outputs and inputs and maybe, if you are lucky, you might connect one address owner to a real person. But in general, I think that cryptocurrency is used only in a smallest part of all shady deal, as cash money are more easy to use and more popular in such areas.
First of all Bitcoin is not anonymous. All the transactions are available in the Blockchain and it is not possible for the users to delete or modify the log entries in Blockchain. And regarding terrorist funding using Bitcoin, during the last 12 years (ever since Bitcoin was first mined), I have heard only two or three isolated incidents (and even these attempts were mostly unsuccessful). The truth is that terrorists don't prefer Bitcoin, due to the difficulty in converting it to fiat cash. Their preferred mode is physical cash.