You can still run an arbitration strategy even with one exchange, and the risk within that single exchange can be different between different coins. For example

You can still run an arbitration strategy even with one exchange, and the risk within that single exchange can be different between different coins. For example, if an exchange's Bitcoin balance is stolen but not their Litecoin, there might be a period during which you can withdraw in Litecoin but not Bitcoin. If the rumor mill works, that would be preceded by a change in the relative prices during which this algorithm would suggest buying Bitcoin.