Why is Vader's ship so bad? and falcon, for that matter.

Vader's ship is probably the best in the game. Yes it has low health and protection but that's because he his basic attacks cannot be resisted or evaded, meaning they go through foresight too. They also always apply target lock and while the ship is active target locks can't be removed.

It works amazingly well with Biggs ship and it's the only ship that can safely take out the imperial tie fighter without causing it to dodge and fed turn meter to the enemy capital ship.

Vader's ship is literally in every team I face