The most irritating bit (spoilers)
is the opening horrific train crash that nearly-but-not-quite kills the cute kids and releases the menace. They look around, find that none of them are dead, and drive away.
It's almost inconceivable that the bigbad US Army would send the menace on a train across the country without a large number of escorting soldiers; it's completely impossible that they could have sent the menace on a train across the country without a driver. Who would then presumably be bleeding to death in the wreckage while the kids bail.
The issue being that the plot brings in incidents on the basis of them being what is called for in the spectacle at that particular moment, rather than on any felt need to conform to what happened ten minutes before or after. Absolute contempt for not only credibility but also consistency.
The menace excavates (with its hands, apparently) a vast burrow beneath the town, through what looks like rock and is certainly self-supporting (not sand, for example, or soil - perhaps clay) and apparently eats the spoil because there are no heaps of rubble left anywhere.
The menace at the end.... let's not even touch on that, the pinnacle of unbelievability is reached, but the issue is if it could do that why it hadn't done it before.
And the kids have to keep their knowledge of the menace secret because otherwise, the teacher tells them, the army will kill them. And at the end of the movie that just evaporates and they're wandering around in full view of everybody and the army is apparently just fine with that. I myself prefer to think that the army did kill them all and that the end-credits movie was put together by their grieving schoolmates, but that's just me.
And again, a vastly overdone subplot about the American obsession about being good fathers, without which no American blockbuster is complete.