By "difficult" the FBI certainly means "we'd have to perform some amount of manual reconciliation because the meaningful data won't just fall into our laps." Pretty standard for the last 15 years.
As for "obfuscation," I was under the impression that the system doesn't attempt to obfuscate what payment addresses are involved in a transaction. It's not pertinent to the network which physical person controls which addresses.
I suppose the question is how recognizable are tumbling services? Do they create new wallets regularly, and does the resulting traffic leave a fuzzy line between what is "in" the service and what is "out"? It could be that using a tumbling service looks fairly similar to giving money to somebody/somebodies who themselves uses the service.
