Having a national emergency notification system - good.

Having a presidential emergency notification system - bad.

Half the people in the US will distrust any message from whomever is president. If the president is a Republican, Democrats will distrust his/her messages. If the president is a Democrat, Republicans will distrust his/her messages.

A national emergency message needs to come from a trusted nonpolitical agency with an impeccable reputation. Of course, there hasn't been such a thing since the Office of Civil Defense was abolished and its responsibilities turned over to the anything but nonpolitical FEMA.