Many students are concerned that the language they use might be too 'simple', so they try to find more 'difficult' words or phrases.
In my opinion, words and phrases are never 'difficult' in the way that a subject like mathematics can be difficult. Anyone can understand and learn a word or phrase. What is difficult about learning a language is building a large repertoire of words and phrases that you can use flexibly to express a wide variety of ideas.
Examiners don't really think in terms of 'simple' and 'difficult'. They look for:
- range and variety of vocabulary
- detailed explanation of relevant ideas
- ability to put groups of words together correctly (e.g. collocation)
- accurate use of small words (e.g. articles) as well as big words