If something is important, one would say its importance can hardly be overestimated/overstated etc. Yet I frequently see the opposite, i.e. underestimated/understated, when clearly it is not meant that the matter is of very little importance. My question: which one is correct, or are both accepted English?
We can't overestimate the importance of this.
It would be impossible to overestimate the importance of this (since it is so important).
We can't underestimate the importance of this.
We must not make the mistake of underestimating the importance of this (since it is so important).
The same principle applies to quite a few words or sentences, out of context. For example:
We can't take this too seriously.
1) This is a joke. We really can't take it seriously.
2) This is very serious. It would be impossible to overestimate its seriousness.
Context is everything in sentences like this.