I will always advocate for a broad and more varied vocabulary. One should use the most accurate words to succinctly convey one’s ideas.
In writing however, the most effective communication occurs when simple language, not simplistic, but simple language is used and ideas are clearly and logically presented.
The core purpose of effective communication is to create understanding in the reader. Simplicity and clarity are features of all good communication.
When a broader vocabulary is brought in to play, then the readership needs to be taken into account. A generally educated readership with an interest in reading will either be familiar with a wider vocabulary, or happy to sit with a dictionary nearby for assistance.
As a general rule, broader readership (newspapers, websites, product instructions, etc.), simpler vocabulary. Writing simple, clear, effective communications take just as much, if not more skill, than a work which allows a more complex vocabulary.
Consider your reader.