I’m quite frankly surprised that the auditors would give such advice. I’ve thought it over, and the only possible negative I can think of is that by default, headings change the size of the font. However, they can be styled with CSS, negating that point.
If it’s not too much to ask, may I know what reasoning they gave as regards this recommendation? I’m curious if there’s something I haven’t thought of, but I am honestly thinking it was bad advice.