In many if not most cases, Cuban players haven't been busts so much as they've been systematically over-hyped during the signing process, which led to unrealistic expectations around Major League Baseball and in the media. The vast majority of Cuba's truly elite players have either stayed in Cuba for their entire careers or left Cuba too late to have a meaningful MLB career.
On the success rate and perception of Cuban baseball defectors in MLB, from the Miami Herald article "Yoenis Cespedes may be the great unknown for Miami Marlins" by Clark Spencer (12 February 2012)
Yoenis Cespedes may be the great unknown for Miami Marlins - Miami Marlins - MiamiHerald.com