They also have to be careful about credibility. It wasn't clear how well the global containment efforts were going to work until very recently. Let's say a week or two ago there's a 33% probability of global pandemic: that means if they call it such then, 2 times out of 3 they're crying wolf and causing people to listen less carefully to the WHO. You already see people claiming all manner of conspiracy theory nonsense and not taking it as seriously as they should; they have to be careful not to amplify that, and balance that against acting too late when it's clear there is a high probability of global community spread.

Other than their censorship in China (they give different advice on their website served to China, even on English-language pages, involving the effectiveness of traditional Chinese medicine against the virus), I think they're doing a mostly okay job so far, from what I can see in the current reporting.