Ignoring the issue of being able to prove system issues with electronic voting machines/tabulation because that is a game changer of such massive scale that it redefines everything.
There are two real legal issues that are somewhat interrelated:
1) Vote-by-mail rejection rates, especially with signature matching requirements. The expected, historical, rejection rate is around 1%, in the states in question this year - despite a ten fold increase (or more) in vote by mail - the rejection rate is a tenth of that.
This is a big issue in Nevada, PA, Georgia, and to a lesser extent WI & MI. Getting intervention from either the courts or election officials to cure the problem is relatively unlikely at scale.
All of those VBM records though are subject to FOIA/Open Records requirements and there are groups already intending to to actual signature matching on at least NV and GA. Those private efforts won't be able to impact the election result simply because of timing, but they are still going to be done and are likely going to result in major scandals before 2022.
2) Failure of state election officials to follow state election laws. This is at the root of the pending SCOTUS case and has the potential to throw WI, MI, PA, and GA (at least) into uncharted territory. All four of those states have implemented procedures (especially with regards to mail in voting) that conflict with the plain text of state election law, and if SCOTUS rules that state election law controls then you are looking at more than the Biden margin of victory in every one of those states being thrown into dispute.
In GA, they would have to re-do the signature match for every mail in ballot as the procedure used was created via consent decree and its at odds with state law.
In PA, every ballot counted without meaningful observer access would have to be tossed. Along with every ballot "cured".
In all four states you would at least have the legal issues resolved and it would become a simple trial of fact. And the initial standard for that is every fact alleged by the plaintiff is accepted as true. Which would mean both discovery and federal injunctions against certifying election results in multiple states.
