one of the criticisms of the Popular vote compact is the lack of a fool proof standard of determining the winner. If the state officials can not determine the validity of a count because of unreliable data from some states, they can decide for themselves who they determine was the national popular vote winner, and states within the compact could end up declaring different winners based on each’s independent review of the data.
The biggest flaw of the NPV compact is that it’s not legally enforceable. There are about 5-6 weeks between Election Day in November and the electoral college vote in December, and the legislature of any state that signed the compact can convene during that time and withdraw from the compact if they don’t like the results of the “popular vote.”