I tend to agree with the positive points that he showed during his Presidency. Particularly with regard to foreign affairs. It was pretty clear by this time that the PRC would be staying for a while longer and losing access to this large market negatively effected the American citizen.
However, the subsequent trade negotiations with China may have put the American middle class under siege. Gone was "made in America" replaced by "made in China" this at least has its roots with the Nixion Presidency.
Reports from the Vietnam war suggested that a peace had been made with the North with the US guarantee.of the South's sovereignty post pull out. The US pulled out and shortly after "not immediate" the North invaded and the US did nothing, de facto losing the Vietnam War. The outcome of the war has proved to be a stain on American history that had geopolitical implication even reaching into the modern Age. Negotiating a peace with the North was a victory, but failing to uphold the guarantee with the South was a failure even if the war was unpopular.
A realm where is he slightly more controversial is in the domestic realm. Nixion was the first President to popularize the "war on drugs" a war that continues to rage this day and has proven to be infamous. It has lead to the continued unpopular decriminalization of marijuana regardless of its medical implications (in many not all states) simply because of inertia. There are numerous and credible sources that he targeted prominent members of his colored citizens in an effort to undermine the growing political consciousness the group has a whole had started to exhibit.
This predisposition to use the government in order to achieve personal political ends was not limited to the blacks. His "list of enemies" is well known and contained the names of personal and political enemies he would have members of his power structure keep an eye on. This implicit "spying" and his paranoia eventually led to the Watergate scandal and while he most likely did not specifically instruct it, the following cover up was a misuse of Presidential power.
Even considering that, he did achieve a number of good domestic policies at home as well. He effectively ended segregation in places it was still quietly in practice as well as headed a number of environmental protection leaps (clean air/water act, establishment of the epa)
He had the makings of a really good President despite some misgivings, but it did seem he acted for the most part with the interests of the American people at heart. Elected with the highest mandate in the history of the Presidency. That is not something bad Presidents can do.