They are similar which means not to mention. “Much less” is used especially in negative contexts and “let alone” is used especially to emphasize the improbability of a contrasting example.

I actually think they are often interchangeable. They both have the same basic meaning.

Hi Cindy Much less: 1. A lot less. We've had much less foot traffic since construction started. This magazine has much less content than it used to. Let alone: 1. verb To stop bothering someone or something. A noun or pronoun can be used between "let" and "alone." Why do you keep picking at that scab on your knee? Let it alone already! For the last time, let your brother alone—he needs to study! I hope it brings more clarity!

let alone- used for saying that something is even less likely to happen than another unlikely thing (I hardly have time to think these days, let alone relax). much less: used after a negative statement in order to emphasize that it applies even more to what you say next (I am no one's spokesman, much less his).