What alternative is there? In my experience, if I'm providing anything other than work paid by the hour or day, then I am promising things I can't 100% guarante

What alternative is there? In my experience, if I'm providing anything other than work paid by the hour or day, then I am promising things I can't 100% guarantee I can deliver. I have every reason to believe I can deliver, but I can always think of some unlikely edge cases where things go terribly, irrecoverably wrong. This bugs me, sure, but the only way I can think of to never get sued for breach of contract is to never sign a contract.