A malicious actor could add the dishwasher to their botnet and conduct DDOS attacks or distribute spam. You could also use the server as a proxy or as an attack vector into whatever network the dishwasher is connected to.
As long as we're speculating, depending on the controls afforded, you could possibly break it or damage a load. Could even be a bit dangerous.
Attacker could lower temperature of washing cycle and make it shorter, enough for a given type of germ or virus to survive the washing, if the thingy is used in hospital and records are kept it should mean that actual sterilization is the main scope.