allegations of bitcoin being used for untraceable money laundering

"It'll be limited to cash operations only."

And how is that a problem(ic significant limitation) in the given context? It seems to me the only fundamental difference between cash, gold, and bitcoin as far as the situation with the criminals and authorities is concerned, is that the criminals have to pay less money to ship fewer kilograms of mass across national boundaries. So it benefits the criminals, but only as a small shipping overhead cost. Not as a cost of being more or less able to evade detection and apprehension by authorities. However I keep seeing implications that this fundamentally is about something different than shipping costs of gold or paper currency.