6 confirms is no guarantee. its just an arbitrary 'acceptable risk' metric that coinbase has chosen. It could just as well be 4 or 13 or 2.

However, if you grok zeroconf, you'll see why it can quite satisfactorily be zero.

If you are receiving a million dollars from an untrusted third party then sure, you might decide you want, say 10 confirmations.

If you are sending $100 to your mate, or buying a coffee, or transferring from your savings paper wallet to your spending bread wallet, or surfing silk road 2 or 3 or whatever its up to now... all of these are pretty much 100% candidates for zeroconf i.e. within seconds you can be sure enough.

In fairness though, there are a whole bunch of use cases in between. That is where it comes down to a bit of personal preference, if you look at a comparable (amount/trust) transaction in the existing monetary system then I would say btc is pretty compelling in terms of speed, though of course you may disagree!

You can insist that you want 6 confirms for *every* transaction, but I don't think its fair to claim that its absolutely necessary, and that this negates the 'bitcoin is fast' argument.

If you draw the chart of "confidence transaction is legit" vs time, even the first confirmation adds relatively little to the already high probability that the transaction is just fine.

Comparable to the chance your $100 bill is fake? Or that you aren't going to get a chargeback? or that a check is going to bounce?

I dunno but it wouldn't surprise me if zeroconf came out ahead in terms of risk. *shrugs*