In my experience it's the opposite, and the client is ususally pretty well aware of whether they are on shakey ground or not before they've brought the consultant in. Quite often that's the very reason why the consultant has been engaged.
If a client wants to employ a consultant to try to get them a result which they wouldnt be able to get themselves - whether they really merit such a result or not - is up to them.
Although many clients are reasonably open and would say that they think they have a difficult case, i'd say it's more likely that the client would keep something from the consultant than the other way around.
You should of course always keep the client advised of the reality of the situation, even if more often than not they'll tell you that they know they haven't got a great case and that's why they brought you in. As a hired gun though, you have no duty to pull out just because you don;t personally believe in the case you are putting forward. You are there to put forward the best case that you can, whether you believe in it or not. That is not being unscrupulous, it's doing the job that your client is paying you to do.
It;s not as easy, I would add, as to say, "settle on this point", as that is always the aim anyway. More likely the alterantives are to carry on in the hope of forcing a settlement, or just give up on it and concede. Conceding is often something that a client can't afford, and therefore the least appealing option.
Remember that determination and dogedness is a very important trait in a claims consultant, and clients don't engage claims consultants to just give up at the first hurdle.