I get silkies with a head twitch when the weather is really hot and they are under stress and lacking something. I pump them full of vitamins and electrolytes as much as possible during that time and after the weather cools off they get better.
Having balance issues or dizziness would not be a TRAIT, but a vaulted skull is considered a trait and yes, it can lead to issues if the birds was injure its head and it did not have a fully developed skull to protect the brain.
Other undesireable traits would be split wing, 4 or 6 toes, red skin/combs, no middle toe feathering, prominate wattles on a bearded bird, any normal chicken feathering( I have seen a silkie feathered bird that HAD the normal feather shape to it- hard to explain) hard feathering on wings and tail, (no shredding in wings)... etc.