I doubt that anyone keeps stats on this issue. My best recommendation would be to look at the case law to find examples in the law.
It happens, but it is very rare and requires a finding of duress, fraud, or unconscionability. From what I have seen, these agreements are most often overturned when there wasn't a disclosure of a significant financial asset.
The Virginia Supreme Court does not keep these kinds of statistics. Instead, it is purely on a case-by-case basis. The factors are whether there was fraud, duress, or similar chicanery in the creation of the Agreement. Failure to completely reveal assets must be of such an excessive level as to equate with fraud. Like so many fraud cases, it's easy to claim, but hard to prove.
For a non-lawyer to try to achieve an overturning of this isn't impossible but as many judges will ask a pro-se ('by themselves") litigant, "would you try to perform surgery on yourself?" Sure, it can be done, but there's a reason why attorneys go through three years of law school, plus many years of practice, in order to be deemed 'competent'. I am NOT trying to be rude to you, but the statistics are truly irrelevant, what matters are the facts.
The last time I tried to overturn one I succeeded in Fauquier Circuit Court in 2005 - haven't tried to do it again until this year, and I have two cases that are presently before the Circuit Courts. When such is overturned, it is a rarity that the case is appealed to the Court of Appeals, and that is where the majority of case law (pre-1985) comes from for divorce cases (unless the CoA is overturned by the Va. Supreme Court).
Talk to a competent domestic relations attorney in your area - the fee for that will far outweigh the potential award of attorney's fees against you if your spouse hires an attorney, and then asks for an award of attorney's fees for a frivolous motion under 8.01-270.1.
Legal issues often depend on the specific facts in any given case or situation. Please do NOT utilize the information you receive as either a binding legal opinion in your case, nor presume that I am your counsel because I've answered a question you had. Any legal representation is accomplished by written contract ONLY, signed by each of us.
As the prior attorneys have very correctly explained, there are no published "stats" on how many times a marital separation agreement has been successfully challenged in Virginia and, even if there were, that information would be of no value to you personally or to your "case". In those instances when such an agreement has been set aside by the court, we only really know about those cases where the court has issued a written opinion and that opinion has been reported, published, or otherwise recorded in a searchable legal database (such as LEXIS, Westlaw, Fast Case, or Case Finder).
What is relevant to you and your case is that a marital separation agreement is a contract and is treated as such under Virginia law. Further, Virginia law presumes that any contract that appears valid on its face IS a valid, binding, and legally enforceable contract. Accordingly, the person who is attempting to challenge said contract has the burden to rebut that presumption -- meaning the burden of proof is on you to properly refute the validity of the contract (for ex, that your signature was forged) or prove that you have a valid legal defense against the enforceability of such contract (such as fraud, duress, unconscionability, etc.).
Since the presumption in the law is that any contract entered into for a lawful purpose and for valuable consideration is a valid and binding contract, the number of reported cases where a court has set aside a presumptively valid contract are rare compared to the number of such cases where a court has upheld such contract. The fact that the cases are reported at all actually skews that number considerably because most of the time when the court denies a motion to set aside an agreement the court issues no written opinion about it to be published or reported. So, if you find that an agreement has been set aside in one of ten reported cases that you can locate, the likelihood is that there are another thousand or more cases where the agreement has been upheld in a ruling from the bench with no written opinion issuing on the case. If a judge is going to set aside an agreement, he or she will almost certainly always issue a written opinion, the other side will appeal, and the Court of Appeals will issue a written opinion about it.
If you want to contest the agreement, then I would urge you to hire an attorney. If you are going to insist on representing yourself for whatever reason and conduct your own legal research, then the best place to start would be the nearest law library you can find. Begin with Section 20-109.1 of Title 20 of the Annotated Code of Virginia (1950), as amended. Both Westlaw and LEXIS publish an Annotated Code of Virginia and, at the end of the statute, you should find “Case Notes”. In the LEXIS (formerly Miche’s) addition, you should find some relevant case law relating to the setting aside of marital settlement agreements under “II Agreements”, subsection “C. Validity”.
This response does not create an attorney-client relationship and is intended for general information purposes only.