Personally, I don't think that criminals are massively using Bitcoin
At least, not big fish and not in the amounts in which cash is used for illegal operations, activities, and transactions. Bitcoin is far from being anonymous (it is pseudoanonymous, at most), all Bitcoin transactions (at least, on-chain transactions which still make up the total majority of transactions) are permanently written on the blockchain for everyone to see and analyze them. In fact, I'm really fascinated why people still consider Bitcoin anonymous, you can't even say that coin mixers are not used by government agencies as nodes to specifically track big Bitcoin transactions
Fiat is even more anonymous since there's no records on who held the paper money you're holding before you got it. It's not transparent and maybe that's the major reason why big syndicates haven't switched to using bitcoins yet. They use paper money since they can't use bank transactions as suddenly having millions of money to your account will raise suspicion.
Only way that you can trace who had the paper money before is by tracing the finger prints. But even then, the only one that you'll see is who had it very recently. Paper money is moving a lot and older finger prints are overwritten by new finger prints.
Fiat will be removed in the next decade or two, therefore what is left for criminals? bitcoin. Bitcoin will be used by criminals worldwide since its the best thing next to cash to do bad things, unfortunately...