The major weaknesses of the Spanish during the Spanish-American War were that they had fewer soldiers and a weaker navy.

As an example of this, the Spanish navy had only 6 ships to defend Cuba.   It also had very few ships in the Philippines.  This was because Spain chose to keep its ships at home to protect the Spanish homeland.  This allowed the US to easily destroy the Spanish navy both in Cuba and at Manila Bay.  This also meant that the US could prevent supplies from reaching Cuba.

Partly because of this, and partly because Spain was not very wealthy, the Spanish had relatively few soldiers, especially in Cuba.  This meant that American forces, even though they were fairly amateurish and very disorganized, could easily defeat the Spanish army in Cuba.

What were the strengths of the U.S.'s enemies in the Spanish-American War?

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.