For example, if you were some random cryptobro buying personal quantities of drugs in 2015 there’s some risk but the FBI probably isn’t going to spend time on something that small. If you do anything which reveals funds going to or from a tumbler, however, the risk isn’t just KYC but also questions like who else used that tumbler - if you were participating at the same time the North Koreans or a big ransomware ring were laundering something, that traffic might be a lot more likely to get analyzed and in the worst case scenario some agents show up trying to get you to convince them that you were unwitting small fry and not hiding more. Lying to them is a federal crime, too, so even if you nothing other than tumble coins for ideological reasons you’d want to be very careful about what you say – and think about how hard it could be to prove you didn’t know the other parties in a transaction.