Given how quickly and easily the US defeated Spain in this war, it is clear that the Spanish did not have many strengths. Their main strengths would have been the fact that they were fighting on the defensive and the fact that they had better equipment on average than the American army did.
Because the Americans needed to invade and conquer Cuba, the Spanish had an advantage. They could fight on the defensive and make the Americans attack prepared positions. The Spanish also had better rifles. Their rifles fired smokeless cartridges, which meant that American soldiers could not see where the fire was coming from. This caused serious problems for the US in the few engagements that actually happened.
These strengths allowed the Spanish to inflict more casualties on the US than they took themselves. However, they were still unable to put up an effective resistance.