[derefr]

You can usually turn these people into productive versions of themselves:

First, teach them how to do informed estimates for the probabilities of these negative factors coming into play.

Then, ask them to point out the probability-estimate below which they won't even bother to bring something up.

Finally, explain that your bar for this is higher than theirs, and request that they only bring things up if they meet your higher bar; for things that meet their bar but not yours, you'll just be discarding what they say anyway, so they'll just be wasting their breath.