I think the mustard complements the richness of the ketchup very well.
My mother once made this. It was really good even though it was only flavoured with ketchup and mustard. Since then I've repeatedly made it. It's nice chilled too, so you can pack it in bentos.
The cuts in the sausages helps the flavours penetrate into the meat. Adjust the amount of ketchup and mustard to your liking (I recommend lots of ketchup).
Enjoy Japanese food around the world.