When you overstate, the reader will be instantly on guard, and everything that has preceded your overstatement as well as everything that follows it will be suspect in his mind because he has lost confidence in your judgment or your poise.
I was briefly tempted to add this sentence to my email signature. It is the sentiment I would most like to convey to people who say things on the Internet. Too often, I read posts from people with whom I actually agree, but who make me flinch by overstating their case and poisoning the rest of their argument as a result. Certainly vet school has drilled into my head the rule that I should never make a statement that I can't back up, but even people who don't have graduate-level scientific training should be able to recognize that it's just bad debate style to make statements that overstep themselves. Less is more.