I absolutely hated every single moment of piano and never really got good at it. It was a different story once I got selected for a chorus in school.
My main struggles with piano were chords, and sightreading. Chords aren't really relevant in vocals since a single person can only sing a single note at any given time, and I wasn't great at sightreading then either. And I find breath control a lot easier than trying to anticipate where my hands need to go (as someone with small hands, a lot of pieces were quite difficult)
There was a similar screening process for the ability to swim, though it was actually for selecting those who could not and teaching them.