That's the whole "are you still beating your wife" rationale you like to quote. You make an unfounded allegation, and then say you need a investigation to determine the facts. Which is exactly what the Biden thing is about. You have to ask with all the corruption everywhere in the world, why is he only interested in investigating alleged corruption of the son of the political rival he consistently loses to in the polls. Witch hunts are okay as long as he is the one pursuing them.
So why was it OK for Obama to launch an investigation of Trump a political rival based on a dossier prepared by Trump's adversary, Hillary Clinton, a Democrat? In any case, the Trump request was rather benign and mild compared what happened to him when the whole secret forces of the US government were directed against him and still are three years later in the current impeachment. Yet, you have no concern with these attacks.
Of course there's political fallout from an investigation. But that doesn;t make the investigation illegitimate. Otherwise, any politician could get away with corruption by saying they're after me for political reasons and the investigation should be dropped, just what Trump has said as well. If you don't want to be investigated, you have to remain squeaky clean, something Biden, Trump, Warren, and all the others really don;t know how to do.