I think Natascha should be up for discussion because TF2 is a game about mobility, and when you can slow someone down to the point of hardly moving at all, the -25% is negligible. Speed is so important for every class so in my opinion weapons like the sandman and Natascha upset the balance of competitive play.
Robin Walker was hinting towards a nerf for the Natascha if anyone read PC Gamer so maybe it will be 'fixed', so to speak, by the time season 4 starts but as it stands now it's debatable.