If you throw a MyError::Foo, I would have to explicitly throw that error

And if I prefix my error messages with "MyError::Foo" you'd have to explicitly throw an error that started with that string to break my error handling.

And if two modules chose bad namespaces--like MyError rather than MyModuleName::Error--then the same problem arises in code that uses both modules.

The error vectors are exactly the same! It's still just string compare.

The solution is in well chosen namespaces--whether error prefixes or execption class names.

Just moving to throwing exception objects rather than strings does not fix the underlying problem: that of badly chosen namespaces!