While technically opportunities exist daily (compare the huge difference in prices this moment at https://coinmarketcap.com/currencies/bitcoin/#markets) but tough to execute due to BTC transfers or fiat transfers being high-friction.
The issue I ran into is you can't move dollars between exchanges easily, so to rebalance, you've got to switch to something else. So you're profiting from volatility on one side and then betting on it not happening when you rebalance... and for the rebalancing, you have to wait a lot longer (as compared to trading instantly between currencies on the same exchange).
The other issue is that you have to anchor to something, and you can't anchor to the USD generally because even dollars in various exchanges aren't effectively worth the same amount (since you can't get them out as quickly / easily / risklessly in all exchanges). So you've basically got to anchor to US dollars on a certain exchange, and make trades based on the value of, for example, "Litecoin if sold on GDAX." That adds complexity, but might just be the reality of arbitrage.
That and I'm not sure the order books were large enough to make anything other than small amounts of money. The "last sell" price is just that--the price it last sold for. It doesn't take a whole lot of money before you've driven the price toward parity (and your arbitrage opportunity away).
Anyway--for me it was just a winter vacation project more for the experience than expecting real success. And it may actually work better on a week that wasn't last week (which was a bit uncommon). I guess we'll see.