I became intrigued by this modal construction recently, after I berated some undergraduates in my comments on their essay that what they really meant was ‘X cannot be overstated’. However, greater reflection leads to me to believe that, while they were probably wrong, the structure is ambiguous.
It turns on the meaning of ‘cannot’, and whether you take it to mean “must not be allowed to be the case” or “is an impossibility that will never occur”. If the former, then “X cannot be understated” means something like “I can’t and won’t allow it to be stated as of less importance than it actually is.” If the latter, then “X cannot be understated” means something more like, “It is impossible to state the importance of X any lower than it actually is” (hidden premise: the importance of X is incredibly low).
The problem with the latter option, is that it is exactly the opposite of what the speaker presumably intends, especially if they meant the former. They might have chosen to construct the opposite statement, “X cannot be overstated”, but it too suffers from the same ambiguity.
I’m still not convinced that “X cannot be understated” means “X is highly important” so much as it means “X is so unimportant that it is not possible to understate it”, but I’m prepared to start giving students the benefit of the doubt, or at least start pointing our the ambiguity of the construction.