The Mitsubishi is a 76 seat regional jet. It is not much of a threat to Boeing or Airbus. However Comac is developing three jets, C219, C229 and C239 that will carry from 170-400 passengers. They will be competing directly with the biggest sellers from Boeing and Airbus. The Chinese airlines rival the American in fleet size and are currently operating Boeing and Airbus aircraft. When the state-owned Chinese airlines are ordered by the state to buy from Comac it will be as large a loss for Boeing and Airbus as if all American airlines suddenly dropped them as customers. Also, Irish airline Ryanair has already signed a deal with Comac, and the prototype has not even flown yet...
As for quality, the Chinese are license producing Airbus A320's. From what I've seen the Comac C200 series is basically a A320 copy.