Nothing in life is guaranteed. Furthermore, even if your agreement is enforceable, the other party still has the right to sue you over it for a judicial determination that it IS enforceable. In other words, even if you win in the end, you can still be sued over it and you can still be forced to incur substantial time and expense in the process of that lawsuit. Many agreements contain a provision that allows the winning party to recover costs and attorneys fees from the loser in the event of a lawsuit, but that does not stop a lawsuit from happening, nor replace the time and emotional expense of such a suit. And, in the end, if the loser doesn't have any money, the winner still has to pay those costs and expenses.
Premarital agreements have additional requirements for validity.
Not in California, at present. The right to waive spousal support in a premarital agreement varies widely from state to state. However, the California State Supreme Court is currently reconsidering the issue and, depending upon the outcome of that case, the rule may be changing in California. At present, however, the old rule still stands. Regardless of any premarital or other agreements, both must do this for so long as the marriage is intact.