You mean "overestimated" too. It's not "weirdness," or "wierdness" is you prefer, it 's stupidity.
I parse the sentence as follows: "We cannot [allow ourselves to] underestimate the critical nature of what we are about to undertake."  It's awkward, true - needs a rephrase, not a wordswitch.
That's not what the sentence means, though.

"Underestimate" as a verb means "To make too low an estimate of the quantity, degree, or worth of."  So swap that into the sentence:

"The critical nature of what you are about to undertake cannot have made too low an estimate of its worth."

In other words:

"The critical nature of what you are about to undertake is worth less than you can possibly estimate."

So the poster really does want to say either "cannot be overestimated" or "should not be underestimated", depending on whether he wants to emphasize its innate importance or our risk of ignoring it.