It's worth reading the linked issue for a refresher on how arbitrage, and this scheme in particular, work: https://github.com/butor/blackbird/issues/100.
I think the pathalogicial case is if the markets never converge significantly, or more likely diverge wildly, e.g. one gets hacked. Otherwise, Blackbird will wait it out until it hits a pre-determined settable profit margin.
I'm guessing plausible return rates are low, but with the relative safety you can risk a bit more than just "crystal-ball"ing the market or playing daily lows and highs.