When to use Service-objects?
Never.
There is of course never an absolute answer to stuff but if you are running it in a background job anyway have you considered directly writing it in a Worker or Job-object? Note that you can always run jobs async when needed.
My main objection against service objects is that all too often they are ill defined as a category. So while having fat controllers or fat models may be a bad thing, just creating a bunch of somewhat arbitrary ‘Services’ is not making the code more manageable.
When considering adding a ‘services’ directory to your app, try to think of what class of problems you want to tackle. And when in doubt, just keep messing around with the somewhat fatter models & controllers.