Accessibility for deaf or hard of hearing users

Going through the accessibility posters from the UK Home Office I found something I can't understand. In the section about Designing for users who are deaf or hard of hearing they recommend to do not use:

• Complicated words or figures of speech.
• Complex layouts and menus.

What's the rationale behind this? How this impairment affects your abilities to read complex text and navigate complex layouts? Are there scientific studies to corroborate this suggestion?