More and more over the years I have come to distrust rules like this. I see it as a bandaid solution. Communicating what is easy and what is difficult is valuable, and not saying just is throwing in the towel. "I can't communicate at well so I'll just be silent". The harder but better strategy is to strive to improve your knowledge. There is also another thing to take away, a sort of principle of caution. When you don't know how hard something is for someone, because it's the first few times you can a certain skill of theirs in action, you should be cautious about assumptions. For instance don't make something out to be easy, and give them a little extra margin. But as you know them better you will have a good estimate of their skill and this will no longer be needed.
Another thing with not saying just, is that it's a strategy for avoiding conflict. Avoiding conflict is a good default, but adopting what amounts to a never engaging in conflict is detrimental. Sometimes conflict is needed. Sometimes you need to subtly or not so subtly tell someone they suck. Sometimes being polite is the kinder move, and sometimes brutal honesty is the kinder move. If someone doesn't know there is a problem then how can you expect them to fix it? If a task should be easy but isn't that's a signal that you have to skill up.