It might not actually matter if one state (or several) decided to break the compact, since the trigger condition for the compact coming into effect is that the compacting states have enough electoral votes to guarantee the outcome of the election.

As long as the defecting state had fewer (or the same number of) electoral votes as the aggregate of the non-compacting states that happened to assign their electors in support of the popular vote winner, then the defection would not actually change the outcome of the election.

This may reduce the incentive for states to defect from the compact, as might the possibility of electors from non-compacting states becoming faithless out of respect for the compact.

Strategically I think the NPVIC shouldn't have been presented as a compact between states but as a set of individual pledges to the people of America that each pledging state would act in the necessary way once enough other states had made equivalent pledges. That would mean the states wouldn't have standing to sue each other, unlike in a compact, but it seems that their ability to do that is constitutionally questionable anyway.

I'd imagine the electors, or quite possibly any voter in the state, would have standing to invalidate joining the agreement whether or not it's actually enforced.