In Morad v. Morad, the Husband tried to nullify the couple's separation agreement because of duress and unconscionabilty. The source of the duress claim? She nagged him so much he just signed the thing to get her off his back.
The Court ruled that 1. The agreement was negotiated by counsel for both sides and was not unconscionable; 2. Waiting three years to try to undo the agreement rendered the duress claim ratified. If your pleading duress you have to do it quickly; and 3. Wife's "relentless pressure" on husband to execute separation agreement did not constitute duress that could support setting aside agreement.
After all, isn't that the reason he wanted the divorce to begin with?