As people have found, threading a slender piece of thread through the eye of a needle requires an steady hand and can often be frustratingly difficult. "Threading a needle" metaphorically expresses that a task must be done with a strong focus, avoidance of distractions and that there is only a narrow pathway (the eye) through to success.

It's a good question, but to be honest, being a 58 year old native speaker of English I've never heard the expression "it's a tough needle to thread." I understand the point, and it could just be a regionalism but that's not an expression I'd ever use or like I said have ever heard used. It sort of reminds me of the Bible verse about a camel going through the eye of a needle (think that's it) and that being easier than a rich man going to heaven, which doesn't make a whole lot of sense, at least not out of context or without interpretation.

The eye of a needle is small. The end of the thread is almost as large as the eye of the needle. Threading the needle requires very accurate aim (and the thread flexes and doesn't go exactly where you aim it). Figuratively, "a tough needle to thread" means "success is just barely possible if everything is done exactly right--but the slightest departure from perfection will result in failure."

It means that some task is going to be difficult.

It means "a very tuff job to perform" in general ....