The thing about mixing is that technically, it should be considered to be an objective fact that the coins were mixed well or not... but in all reality it comes down to the fact that it's actually very subjective.
The mere fact that with bitcoin it is technically possible for people to see that coins came from wallet A to wallet B presents a massive problem, whether you want to see it that way or not. Coinbase, or any other exchange, are technically able to shut down your account on the mere assumption that you received coins from a wallet they deem as "sketchy"... that is a major problem in the fundamentals of bitcoin, or any decentralized currency system. Blockchain analysis will only get worse as time goes by, and if we rely on centralized sources as exchange sites, then we will see major issues with the on/off ramp to bitcoin<->fiat. Having/holding bitcoin as a store of wealth out of the fiat system is one thing, but to say that it can be used as a fungible currency is nonsense.