Most tumblers' implementation are flawed, allowing the senders and receivers to be identified. AFAIK the attack can only be prevented by forcing every participant to use multiples of a common, low entropy amount (ie. 0.5BTC), which most mixers don't do
In practice, the people using those services are ones that are primarily on the darkweb. So you're just exchanging your own tainted bitcoins for someone elses tainted bitcoins.
The point is to break the chain of evidence tying you back to the darknet transaction, which works, but still manages to look suspicious as hell.
IMHO, it's only a matter of time before someone is convicted of money laundering for using a tumbler service.
