I guess it depends on the audience, Ian, and on how common the words are. If I am talking to clients, then I do not want to run the risk of being misunderstood. I tend to use simple words and simple sentences. If I were lecturing in academic circles, I might feel different.
“Shiitake mushrooms” and “hello” are in common use, and there is a whole extra syllable (or even a whole extra word) on the end. “Niggard” has only that hard-to-hear d on the end.
Furthermore, “niggardly” is a word that, in my teenage years back in the 16th Century, usually appeared on SAT-practice tests, meaning it was NOT a common word. Those tests rarely asked about words like “table” or “chair”, but “niggardly” and “foppish” could be counted on.
Don’t get me wrong, I understand your perspective, but I think that a speaker needs to take into account his/her audience and context. You ask how the speaker can be responsible for what his audience hears, and I agree that is somewhat beyond his control. But I also think that he should go to whatever lengths are necessary to be sure that his communication is clear.
Forget Shiitake and hello, let’s imagine you work for a car company and you’ve come up with the name “Nova”, a perfectly good car name in the U.S. But your great idea to market in Latin America fizzles because they Spanish speakers heard “No va.” You cost the company millions of dollars. You believe that you shouldn’t be held responsible because the listeners didn’t comprehend what you said?
The other famous story is the speaker to a bunch of high school kids in Brazil, who were coming to America on an exchange program. The speaker wanted to tell them that everything was going to be OK, so he made the “OK” gesture (thumb and finger making the letter “O”.) That sign was read by the kids as meaning there would be plenty of loose women in America. Are you saying the speaker shouldn’t be held responsible, because “what he said” and “what they heard” were different?
Examples can go on and on. Sadly, there’s no surefire way to prevent miscommunication, but there is a process that can reduce the risk of miscommunication: thinking about what you are about to say, and then putting yourself in your audience’s shoes and asking what they will hear.