There are several reasons to expect that a broad pre-emptive pardon would be
held invalid.
A pardon which doesn’t specify in sufficient detail the offenses pardoned conflicts with
the notion that accepting a pardon is an implicit admission of guilt. It is hard to say
that a person who has accepted a pardon for any offense he or she may have committed
has admitted guilt to an offense when we don’t know what wrongful behavior has been
acknowledged.
Moreover, judicial language justifying pre-emptive pardons appears to have pardons for
specific offenses in mind. In addition, it is unlikely that the framers would have vested in
the president the power to issue pardons for offenses of which he was unaware.
Amnesties which can benefit people whose identities are unknown must nevertheless specify
the offenses that are forgiven. Nevertheless, the question of how specifically a pardon must
detail the pardoned crimes remains open, and the Nixon pardon, which has been generally
accepted as legitimate, may figure importantly in any judicial discussion.
https://www.brookings.edu/articles/presidential-pardons-settled-law-unsettled-issues-and-a-downside-for-trump/In other words, the president should have knowledge of the crime/s the person committed, or else he's just writing blank "Get out of jail free" cards. Given that is the case, what would happen if a blanket pardon happens to coincide with violent and vile crimes the president had no knowledge of? Seems like a very negligent and careless use of such a powerful authority.
Also, by accepting the pardon, the person is tacitly admitting they will need that pardon should their crimes ever come to light.
What Biden has done is a monumental abuse of his power. While he might be legally within the Constitution to do it, the way in which it was done erodes the justice system, faith in government, and allows corruption to flourish under the knowledge an indescript pardon avoids an investigation, charges, trial and sentencing. Basically, it's a shield to avoid legal penalties for any criminal acts committed in a given timeframe.
The only part of this scandal that is positive is Biden can't pardon anyone for crimes committed in the future. So, any laws broken after the pardon's effective date are fair game. if those crimes happen to be associated with pardoned crimes, the DOJ ought to be able to dig those crimes up as well based on possible conspiracies, on-going criminal activity and evidence of a criminal organization -- as long as no charges are filed for the pardoned crimes.
I still think immunizing all those people against prosecution is going to bite the Bidens in the butt at some point. All it takes is for someone to get caught breaking the law after the pardon's date, and they'll be happy to spill the beans in order to save their own skin.