Wobbling would be more influenced by the caster angle than anything (which is really the rake angle on a bicycle or motorcycle). It’s the same reason cruiser motorcycles are easier to handle than sport bikes at lower speeds.

Straight bars would be more responsive since they’re a shorter lever, while the curved bars are a longer lever, meaning it takes more arm motion to turn the wheel, so those bikes should be more stable/less twitchy.

Must be more at play with the wobbly riders you’re seeing.