The problem is not only finding someone who truly deeply understands the language, but knowing how to tell that they do.

I've seen way too many "Senior" developers who really have just done the same junior-level coding for 15 years in a row. These people also tend to have a poor grasp of where their skills really stand.

People reading HN probably are self-directed enough not to fall into this trap, but we are not the majority of developers.