Off the top of my head, I can't think of any expression that is similar to "rare as hen's teeth."

I don't think I'd be confused about what that expression meant if I heard it though. Same with "it's like pulling teeth."

Both seem very self-explanatory, whereas, if you look at the worms expression, you don't automatically think that it means it was hard to get information from the guy.

I seems odd to create an expression that isn't readily understandable.