All deals start off based on assumptions which some times don't survive reality. If you have empirical information that contradicts those working hypotheses you initially made then by all means speak up!
Just to let you know my own experience in this area, some colleagues of mine worked for months on a deal (c. $500MM) and were close to signing when someone asked me to sit in on the meeting. To this day, I can't remember who! Very annoying. Anyway.... We must have been 25 people - buyer, seller, bankers, lawyers, advisors,... - with only a few talking for all of 5 hours. You know the kind of meeting, I'm sure. Just before we were going to break the lead negotiators on both sides speak up and say, "Okay. We agree on $500MM and we are moving forward with deal." At that point, realizing how screwed up the numbers were I couldn't shut up any longer. So I spoke up and very calmly explained how the assumptions underpinning the valuation were completely questionable and there was no way we would ever pay that amount for the company. That was it. The deal was dead in the water. As a result, we had to write-off $750MM of previous purchases that had been done by my predecessor and 3,000 people lost their jobs that day. I am certainly not proud of putting so many people out of work but I was angered - and still am - by the incompetence of those who did the crappy job that put all those people and their families in harm's way! This incident motivated me to leave the world of big business.
For goodness sake, speak up when you know its bad deal. If you understand why its bad - what everyone else isn't seeing - you can say it simply, and everyone will open their eyes. They will have an "ah ha" moment and make the best decision.