as far as things showing up on sea-trial, not too much, really. But some things like bent shaft, prop, engine reliability (cooling/knocks/bangs/rattles) can come to light.
A decent sea-trial (at least an hour) will tell me how an owner treats the boat. Does he back away from the slip without letting the engine warm up? Does he bang her from reverse to forward? Does he haul the sails up with effort or does he yank on the halyards till they creak with stress? Is the gear stowed, or is stuff flailing all over? does he check the thru hulls? How does he sail the boat... with care, or, point it in a direction, let the boom slap from side to side, and the jib get fouled up in the rigging.
Things like this tell me more about how a boat was treated than a walk-thru.
Is one needed?... on a 30 ft for 20k, probably not, on a 40 ft for 100k, you bet.
We are not primarily on earth to see through one another, but to see one another through
Some people are like slinkies: not really good for anything... but you can't help laughing when you push them down the stairs