This I disagree with. Telling someone that something is hard before you teach it to them is a self-fulfilling prophesy.
My wife is a native Russian speaker, and she is slowly teaching me Russian. I struggle when learning new languages, so it's easy for me to get discouraged when the progress seems too slow.
My wife often teaches me a few words at a time, and without fail, when she says, "This word is hard," before teaching it to me, it takes me the longest to learn those words.[0] It's like my brain, upon hearing that something is going to be hard, assumes that I'll forget it anyway, and simply turns off.[1]
If you are getting ready to teach a difficult topic, perhaps spend a little more time beforehand trying to find good analogies and approaches. Oftentimes, a topic's difficulty melts away when a student can make an appropriate connection.
Don't confuse the difficulty of teaching a topic with the difficulty of learning it.
[0] You might argue, "Those words probably are harder." That's a fair point, but in the hours before our wedding, I managed to learn how to say, "Я знаю что я никогда не буду голодным. [I know that I will never be hungry.]" On the other hand, it took me days to learn, "карандаш [pencil]" after she told me it was a hard word.
[1] When I recognized this phenomenon I asked her to please stop telling when she expects something to be hard.