Disembarking when a vehicle is wrecked is
not an emergency disembarkation.
Rules page 80 wrote:Wrecked. The passengers must immediately disembark in the usual manner (seepage 79), save that they must end their move wholly within 3"rather than 6". If,even by performing an emergency disembarkation, some models are unable to disembark, then any models that cannot disembark are removed as casualties. This does not prevent the rest of the unit from disembarking.
{emphasis mine}
Rules page 79 wrote:Emergency Disembarkation. If any models cannot disembark, because of enemies or because they would end up in impassable terrain,the unit can perform an emergency disembarkation.
So if any models in the unit were forced to make an emergency disembarkation, they would be placed in base contact with the vehicle's hull,
rather than the access points and then move as normal for a disembrkation. This would indeed prevent the unit from performing any voluntary actions for the rest of the turn. But a raider is open-topped, so the
normal access points for the vehicle
are the entire vehicle's hull, so it is doubtful the wyches in this case would have to perform an emergency disembarkation!