One of my main criticisms of typical “acapop” arrangements comes down to this question:
Would it sound better if those parts were being played by instruments?
So much of pop a cappella consists of straight lifts from original band recordings, and minimal adaptation to vocal writing. The problem is, that guitar part will never sound better in voices than it will on guitar.
It’s only by transforming the music, so that it is truly idiomatic to voices, that you will create a compelling arrangement. Otherwise, you’ve just got a weak cover with different instrumentation.