Nuclear-thermal's Achilles heel is that you can't use it to boost off Earth. So you need a high thrust chemical engine to get it into space. It's hard to make the numbers work -- nuclear-thermal may be more efficient than that chemical engine, but it's dead mass while you're boosting it out of the Earth's gravity well. If instead you can use the same chemical engine to get to Mars that you used to boost yourself out of Earth, you don't have that dead mass. If you want to get to Mars fast, just boost some extra fuel on an extra ship.
Nuclear-thermal looks nice on paper; it's probably the highest efficiency high thrust engine that's achievable with today's materials. But it's hard to find a good use case for it. A manned Earth->Mars run used to be it; but SpaceX's plan for in-space-refueling shortens the trip time just as much, if not more.
But don't take my word for it, there are 12 pages of intelligent comments on the Ars article, comments mostly of much higher quality than any comment here, including and especially mine.
And those comments are fairly disparaging. Unless you're going past Jupiter, in which case Nuclear-Electric-Propulstion is your best bet. And nuclear-electric is very different than nuclear-thermal.
And inside the orbit of Jupiter, solar + ion engines are way more efficient.
The Ars commenters are very well informed, they go back and forth on this a lot. I highly recommend reading the comments there.