That such leaders were collaborating with communists supported by the USSR or by Cuba is not the same thing as accusing them of being spies.  Such collaboration was open, public knowledge, usually by the leaders themselves, just like the wave of Latin American "21st Century Socialists" openly collaborate with Cuba, Iran, and Russia during the past decade and up to the present.

So, which, if any of such leaders were actually falsely accused of being foreign agents of Soviet intelligence, which would support your argument that US intelligence agencies are actually bad at discovering such evidence?