Some version of "you never get a second chance to make a first impression" is used by every graphic artist.
They just don't follow up on the theme by explaining where they came up with the figure. Consequently it's like any baseless claim ...unless you add the context, the "reason why."
Paradoxically, this is not a bad appeal ...because so few designers prove out design elements the way copy is proven. (That's testing)
...What one design change boosted response 354% without changing one word of text?
...Does your web site pass the "credibility test"? Ten design tips which can boost a web site's credibility -- any why most graphic artists won't know them.
Unfortunately, in the real world, the last person you'll get the answer from is a designer. They talk a good game, but don't deliver the goods. Show you're the exception -- with proof -- and you then position yourself against all the "first impression" happytalk.
Related:
WHY Does Design Matter? Pay close attention to the wording. Because most designers insist 'design matters' as if it were carved in stone. Trouble is most graphic artists can't tell you why design matters, simply that it does.