On the HUAC in hollywood

HUAC and Censorship Changes

Same author defending the first round in Hollywood

Wiki

Certainly very little propaganda made it into their films. Only one film, Mission to Moscow (1943), was ever found to have any traces of such influence, and it was produced as much out of enthusiasm for the Soviet Union's role as an ally in World War II as out of Communist influence.
The point here was? Especially after the "Hollywood Ten" 1947.

From 1951 to 1954, HUAC, now directed by John S. Wood, again focused on Hollywood, compiling a list of 324 present and former Hollywood workers who supposedly were or had been members of the Communist Party. Whether or not these people admitted membership, they ended up on an unofficial blacklist.
The issue isn't the detection of Soviet spies, but the method's used.

That they didn't investigate the KKK as

The HUCA originally investigated both left-wing and right wing political groups. Some called for the leaders of the Ku Klux Klan to be interrogated by the HUAC. Martin Dies however was a supporter of the Klan and had spoken at several of its rallies. Other members of the HUAC such as John Rankin and John S. Wood were also Klan sympathizers. Wood defended the Klan by arguing that: "The threats and intimidations of the Klan are an old American custom, like illegal whisky-making."

Eventually Ernest Adamson, the HUAC's chief counsel, announced that: "The committee has decided that it lacks sufficient data on which to base a probe." John Rankin added: After all, the KKK is an old American institution."
Link

doesn't exactly shine well on this organisation either, as it's a failure to do thier job.