The difference between a lot of these and the 'over/under-estimate' thing is that you always know what someone means when they mis/use the others - with 'can't be over/under-estimated', I find sometimes, as in the above instance, that I have to stop and think for a moment before I get the point. If you aren't familiar with the subject at hand, or haven't been paying close attention, you might not know if you're supposed to think that the matter is of utmost import or if you are being advised not to take it too seriously. "The danger cannot be overestimated" - okay, so do I take extreme precautions, or do I relax about the whole thing?