[xorcist]

That's a very good question! It's robust in the sense that there is a lot of work produced with no downtime, but not very in the sense that most people gets less useful work out of it. Consider the case there most users would like to tumble their coins before each transaction, as was the case with certain markets, preferably a full 16 rounds and how that would affect usability of the system as a whole. This problem affects dark net markets just as well as exchanges.

There are also other real life examples too, of single wayward services that made the blockchain harder to use for everyone, such as the casino which recorded a transaction in the global ledger for all eternity for every dice roll. Those are all priced out now, together with some of the more excessive tumblers, but that solution could be considered crippling the network.

Are there "too big to fail" services within the Bitcoin ecosystem? Matters are complicated further by the fact that the exchange in question foot the bill to their customers, one customer at a time, so the economic impact for them is negligible.

Why "fail", then? Well, it turns out that somebody looked at their hot wallets recently and noted that the mostly inappropriate choices of transactions to make had resulted in a non trivial amount of non-spendable dust collecting. In a bank run situation, that would result in them not being able to fully empty their wallets. The further down we investigate, the more there is to learn.