Much gush has been written about Robert Carl (Bud) McFarlane, the president's departing national security affairs adviser. He has been lauded, even by his critics, as wise, patriotic, hard-working, fair-minded and effective. President Reagan said of McFarlane, in words appropriate for a eulogy, that "few have served with more dedication, none with more loyalty."

What is remarkable about this litany, in an administration rarely given to understatement, is that it is largely true. On those few occasions when the Reagan administration decides to rid itself of an embarrassment, the appointee usually is sent packing wreathed in praise appropriate for St. Francis of Assisi.

But McFarlane, despite the praise, remains curiously undervalued, and not just because the Reagan administration has debased the currency of personal commendation. He is undervalued because his best work, on some of the administration's most difficult and sensitive missions, was done out of the limelight but with a quiet skill that earned the respect of colleagues, Congress and the media.

Almost by definition, McFarlane's job was impossible. In an age when a single act of terrorism can humble the greatest superpower, the national security affairs adviser is always on call. When he errs, he lacks the vast institutional bureaucracies that buffer secretaries of state and defense and counterattack against critics. After two years on the job, as William P. Clark learned before McFarlane, there are nights on which the national security adviser cannot sleep, even when there are no calls.

McFarlane was no mastermind. He miscalculated, along with others, the cost of U.S. involvement in Lebanon. He sometimes played his cards too closely to the vest in his bureaucratic dealings with powerful Cabinet members. He worked too hard, sometimes to the point of exhaustion. He did not suffer fools gladly, even when they outranked him.

But McFarlane also provided the administration with a window on the world that it had sorely lacked and badly needs. He was the administration's early-warning system, spotting trouble before it became uncontrollable in distant nations or on Capitol Hill. He was also a one-man liaison with the disparate communities of political Washington. When the Central Intelligence Agency wanted to make Reagan aware of the deteriorating situation in the Philippines, it relied on McFarlane to persuade the president of the necessity to put pressure on his longtime ally, Ferdinand Marcos. When moderate Democratic members of Congress wanted a Reagan commitment to genuine arms-control negotiation as the price for supporting the MX missile, they called upon McFarlane to carry the message. He also took the lead within the administration in pressing for limited economic sanctions against South Africa, realizing that Congress would act if the president wouldn't.

My views of McFarlane's importance are formed more by his character and style than his policy positions. In dealing with reporters, as with Congress and his colleagues, he was inevitably an educator who expressed his views with precision and reflectiveness, thoughtfully listened to opposing viewpoints and understood the value of disagreeing in an agreeable manner. He acknowledged mistakes. He has, in the fine words of the Founders, "a decent respect for the opinions of mankind."

McFarlane was not self-inflated with the importance of his position. He told self-deprecating jokes, recalling that when he first appeared on television, a young employe of the network observed that he had "the most boring face" she had ever seen. He joked about his Marine Corps background and his sometimes meandering briefings. He did pluperfect imitiations of his former boss, Henry A. Kissinger.

At times, McFarlane's "no comments" or failure to return phone calls exasperated those of us who make our living in journalism. But we will miss him a great deal. Reagan and the country will miss him even more. Reaganism of the Week: Speaking to students at Fallston, Md., High School Wednesday, the president said he had told Soviet leader Mikhail Gorbachev that their task would be easier "if suddenly there was a threat to this world from some other species from another planet outside in the universe. We would forget all the little differences that we have between our countries, and we would find out once and for all that we really are all human beings on this Earth together."