I would like to modestly propose a rule of thumb: only use words that your readers are likely to understand unless you define them yourself. The problem with using obscure words and phrases is that most readers will probably not bother to look them up, inferring the meaning instead from context. This is bad for two reasons. First because if you have carefully chosen a word for its precise meaning and connotation then it is a waste to have people insert some generic and colorless place-holder.
Second, this sort of thing is the way that language deteriorates. I once had a friend who used the word "literally" to introduce non-literal superlatives. "I would have had to read literally millions of lines to get to that part!" --where he really meant somewhere on the order of a thousand lines. Clearly he picked this up because he inferred the meaning of "literally" from its use rather than looking it up. I understood his difficulty and in the interests of amity, I managed to avoid strangling him for the five long years that we worked together, although he made this doubly difficult for me because he also demonstrated a fondness for the barbarism "irregardless".
Of course there is an exception to the rule I suggest. You can occasionally use a rare word in such a way that you expect the reader to infer a correct meaning even if the word is unknown. In this way you can shine sunlight upon serviceable words that otherwise languish undeservedly in the caliginous corners of the language.
Exercise for the reader: what do you think "caliginous" means? No looking it up until after you answer!