I'm not sure that this blows the client away tbh. I had much more success with being critical but within reason. If a client suggests a feature I do not believe will work or is not worth the time, my approach is to agree that this is technically possible and it will take X amount of time with Y drawbacks (e.g. harder to maintain in the future, manual effort later on). And then I'll tell them my opinion about it from a business perspective. I don't push in any direction and make it clear that if this is what they want, I'll get to work right away.
But then again, I only work with clients who appreciate feedback and don't think of me as a person turning coffee into code ;)
But then again, I only work with clients who appreciate feedback and don't think of me as a person turning coffee into code ;)