‘Ex Parte Garland’ and a couple other cases are the reason the Ford pardon of Nixon was never seriously challenged.
The power of pardon is rooted in common law and original intent. Were it being newly considered by the current lawless courts, it might not stand, but now there are long-standing precedents and significant historical actions connected to it, so it will remain nearly absolute.
Flagrant abuse of presidential pardon could always result in impeachment.
The distinction is crimes committed during war time versus crimes committed during peace time. During war there is a break down of civil authority and general chaos. During the chaos crimes will be committed due to circumstance. During peace there is civil authority and general order.
Currently the extraordinary circumstance of war is absent and is not a mitigating factor which warrants a blanket pardon. This is also true in the 1974 pardon of Nixon.