They have a history of "disappearing" once they reach other countries.
Cuba is a very poor country, and the average wage of doctors was just around $50 a few years ago. Compare that to the average wage of $500,000, which most of the doctors receive in the US. So they have a valid reason to defect. Raul Castro recently increased the salary of the (overseas) Cuban doctors to around $1,000 per month, thereby discouraging the defections. But still, a few will happen anyways.
But their intention should be appreciated. They did whatever in their capacity, to help these West African nations.