Hi! I just had to check out your mod because one of my own mods (Autodrive) has something similar to your recoil effect: it lets vehicles bounce on collision. Something I've noticed in your function apply_recoil():

  -- Extremely horrible way to get the target position of an effect
  -- No, event.target_entity.position doesn't work. It just gives the source position.
  local shooting_state
  if source.type == "character" then
      shooting_state = source.shooting_state
  elseif vehicle_types[source.type] then
      local driver = source.get_driver()
      if driver == nil then return end
      if driver.type ~= "character" then return end
      shooting_state = driver.shooting_state
  end

Autodrive handles vehicles based on car and on spider-vehicle prototypes. When a vehicle is autodriving, there may be a player or character inside -- but it is not guaranteed. If no player has entered the vehicle, Autodrive may create a dummy character and use it as vehicle.passenger. This happens when the vehicle is following a path where it must open gates, or if it has sensed enemies within range of its selected weapon (if that has ammo).

Your mod will miss auto-driving vehicles that have a dummy passenger but no driver. I suggest the following change to fix that:

  local shooting_state
  if source.type == "character" then
      shooting_state = source.shooting_state
  elseif vehicle_types[source.type] then
      local shooter
      if source.type == "car" or source.type == "spider-vehicle" then
        shooter = source.driver_is_gunner and source.get_driver() or source.get_passenger()
      else
        shooter = source.get_driver()
      end
      if (not shooter) or shooter.type ~= "character" then return end
      shooting_state = shooter.shooting_state
  end