The only valid attack on Bitcoins fungibility would be on the number of confirmations a UTXO has.

A UTXO with 1 confirmation is harder than any Fiat money but not nearly ad hard as a UTXO with 2 confirmations.

The difference in hardness decreases towards zero as number of confirmations increases.

If the medium of exchange is Lightning then liquidity channels are based on UTXOs who have exceeded the threshold and then all money moved across those channels are perfectly fungible: