Your clients hire you for your expertise, not to agree with them. If their design approach is clearly flawed, tell them. Better to be unpopular, or even lose your assignment, than to help kill the client’s company.
Corollary: Clients hire you for your advice, but that doesn’t mean they have to agree with you. If your recommendations are ignored or overruled, as long as you’ve provided your best advice based on the data the client provided, your task is complete. Just smile, deposit your fee, and move on.