Why wasn't Ford's premature pardon of Nixon challenged?

Why wasn't Ford's premature pardon of Nixon challenged?

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It was.

The Dem-GOP deal on removing nixon ruled that out

It was, but many people believed it was better not to drag a former President through criminal proceedings.

How was it premature? Article II, Section 2 of the United States Constitution states that the President "shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment". Nixon has already resigned so he was not subject to impeachment.