I wish Janet used Option types instead of using nil. No idea why modern Lisps don't borrow from Rust's goodness.
They are an everyday falsy value that you program _with_ not around. Nil doesn't signal that you forgot to initialize a value. It simply means stuff like "I don't have this" or "no more". You don't check for it at every corner like an anxious squirrel, but pass it around freely, knowing that your code knows, accommodates and embraces nils.
We used them at work (Clojure) and it didn't solve anything; normal idioms don't apply to monads and the mental overhead of having to know when they are used is the exact same as having to know when something might be nil.