I am going to address the folks treating the CBA as ironclad and that the NFLPA has no chance or that they have zero grounds or merit in a lawsuit, or are reading articles from lawyers saying the NFLPA has no chance:
As a matter of law that assumption is incorrect. CBAs are constantly challenged in court and lawsuits have been successful in the past and there is never "no chance." In particular discrimination lawsuits have been successful (though have a different legal test).
CBAs, and processes and outcomes coming from them, can absolutely be challenged.
But first I want to say this: Any lawsuit that is appealing an increase the suspension from like six to eight, or ten games, is probably wasting their time unless it has definitive proof of something corrupt. However, if Goodell comes down with a year, or indefinite, that is completely different because Courts in the past have shown much deference to the decision of the Arbitrator. Much will turn on why Goodell changed Robinson's finding so drastically (if he does).
Generally speaking of employment law case law, a ruling from a CBA will prevail, and the Courts will defer to it, unless the plaintiff can demonstrate that the decision by the arbitrator, or the final appeal authority, was 1) Not properly followed, that is a meaningful defect in the decision-making process 2) The process was corrupt or fraudulent, that is to say it was pre-ordained or imposed for reasons outside the scope of the case, 3) Is outside the scope of basic principles of the law, or lies well outside the scope of the intent the parties agreed to in the CBA. 4) Is a punishment so outside the scope of the norm as to constitute an undue burden.
And I will add another, not based on case law, but a feature of the CBA itself because of the clause that the owners and players must receive equal treatment and held to the same standard 5) That the punishment is so onerous and unprecedented that it can be categorized separate treatment than that imposed on other players or owners in similar situations and violates the CBA.
Now it is interesting because Robinson's finding gives both the NFL and Watson something to stand on. The NFL can stand behind Robinson's finding, according to Law360:
Robinson found that despite Watson's denials, the league met its burden of showing that he intentionally made sexual contact with the women and that he knew it was unwanted. She said that fit the league's definition of sexual assault as "unwanted sexual contact with another person," and that the NFL also carried its burden of showing he violated personal conduct policy prohibitions against conduct that threatens the safety of another person and that undermines the NFL.
However, within her ruling is also a kernel upon which the NFLPA can make a case, based on my point 5), and if they have evidence, Goodell in overruling Robinson is grounds for a determination based on the merits on points 2), 3) and 4).
However, Robinson noted that the NFL itself characterized its punishment of an indefinite suspension of at least a season as "unprecedented." She said players accused of violent acts have generally received six-game suspensions and that the allegations levied against Watson do not fall into that category.
Players accused of nonviolent acts have received at most three games, Robinson said, and that it would be unfair to dramatically increase the punishment for such conduct without warning. She said six games was appropriate given the egregious nature of the allegations and Watson's "lack of expressed remorse."
In a footnote, Robinson also cited the NFLPA's argument that it would be unfair to severely punish Watson for violating the personal conduct policy when team owners, who are also subject to the rules, have escaped punishment for "similar or worse conduct."