Is this because Yaml doesn't whitelist the classes for the objects that may be instantiated? They are allocated and then instance_variable_set'd so I'd be Very interested to learn how this poses a risk.
If I implied that I doubted them, then I failed to communicate my point effectively -- I am very curious about how to turn a class allocate + instance_variable_set into remote code exec. I see how you can create the arel objects for sqli, but not arbitrary ruby.
In much the same way that letting attackers control the parameters to fork() would be a bad idea for a C program or letting attackers control the parameters to Runtime.exec() would be a bad idea for a Java program.
This is a Rails vulnerability, not a Ruby vulnerability.