Some adages apply, I think: Clients get the advertising they deserve. Clients who get the very best advertising are those who refuse to settle for anything less.
Too many clients fail to realize that often, they are not part of the target audience. So, what appeals or does not appeal to them is irrelevant.
Further, too many agencies care nothing for whether or not the ads are effective, but only for whether or not they please the client. In B2B advertising, especially, this is almost the norm.
The "how" of evaluating the effectiveness of ads must be worked out between client and agency BEFORE the ads are run.
Many agencies I have seen don't want to know if the ads worked or not, because it might trample on some of their cherished notions, especially about design and typography.
Many clients don't really care whether the ads worked or not, because they are concerned only with pleasing their superiors.