Al Capone wasn't prosecuted for tax evasion because it's cool or smart or strategic to prosecute murderers for lesser crimes, but because proving murder in court was going to be more difficult.
That's not analogous to a Congressional impeachment, but the opposite of how Congress operates. Congress sits on indisputable evidence of the greatest crimes while impeaching presidents for lesser offenses that are harder to prove.
Andrew Johnson publicly did everything he could to limit "freedom" for African Americans to a meaningless word. He was impeached for firing the Secretary of War.
Richard Nixon had indisputably bombed Cambodia, a crime that one failed article of impeachment charged him with, not to mention Vietnam and Laos. In fact, he had sabotaged the peace process and kept a war going for years during which millions of people were killed. Lyndon Johnson (who had committed similar horrors) believed Nixon guilty of treason for the sabotage. When Nixon fled Washington, he was about to be impeached for employing a group of thugs to break into a Democratic Party office.
Bill Clinton had, like Nixon, launched a war in violation of not only laws but also of the will of Congress. Lesser charges could have included numerous disastrous policies that haunt us still from trade to banking to media to prisons to welfare. Congress impeached him for lying about sex.
Donald Trump came into office openly violating both emoluments clauses, threatening to wage wars to steal oil and kill people's families, promising to discriminate in immigration on the basis of religion, inciting violence at events, fighting vote counting in courts, and committed to politicizing prosecutions. Republican Congress Members are upset that RootsAction.org advocated impeachment on inauguration day. I hadn't wanted to wait that long. Trump has publicly threatened nuclear war on two countries, waged and escalated numerous illegal wars, and dramatically increased the drone murder program. He's abused the pardon power and the power to declare emergencies. He's promoted racism and hatred. He's separated children from their families. He's illegally torn up treaties and proliferated weapons technology to brutal dictatorships. He's intentionally exacerbated climate collapse. Congress has ignored indisputable public acts, and impeached Trump for demanding information about a political opponent while delaying a gift of money to Ukraine to buy U.S. weapons.
Of course in their rhetoric and through the media many Democrats have claimed that Trump secretly works against the United States on behalf of the Russian government and that Russia somehow elected him. If there were any evidence for such things, they would constitute a serious reason to impeach Trump and remove him from office. But there actually is evidence unquestionable evidence of numerous other outrages, and Congress isn't interested.
In three presidential impeachments, Congress will achieve three somewhat intentional acquittals. Impeachment will be made to look weak and useless. People will be reluctant to give Congress a fourth swing at it.
But look at what's been passed up without a word. Look what we're supposed to imagine: that Barack Obama did nothing whatsoever that was impeachable, that George W. Bush committed no impeachable offenses, that every president going back through 45 of them with these few exceptions was undeserving of accusation.
Congressman Dennis Kucinich introduced 35 articles of impeachment against George W. Bush, not a one of which interested Nancy Pelosi, even though polls found a majority of the U.S. public in support of an impeachment that wasn't even happening.
Launching the endless wars, imprisoning without trial, torturing, mass warrentless spying, secret laws, signing statements, domestic use of the military, and dozens of other outrages just don't measure up to bribing Ukraine for dirt on Joe Biden (who himself publicly brags about bribing Ukraine for other purposes). Or so Congress would have us believe.
Impeachment has not been overused. It has been underused and misused. It and we are the worse off for it.