NTR have high specific impulse but relatively low power to weight, this makes them good in space and poor for getting out of the gravity well as discussed here.

NTR have high specific impulse but relatively low power to weight, this makes them good in space and poor for getting out of the gravity well as discussed here. They are efficient at using reaction mass but not for power to weight. From the article: Early publications were doubtful of space applications for nuclear engines. In 1947, a complete nuclear reactor was so heavy that solid core nuclear thermal engines would be entirely unable[23] to achieve a thrust-to-weight ratio of 1:1, which is needed to overcome the gravity of the Earth at launch. Over the next twenty-five years, U.S. nuclear thermal rocket designs eventually reached thrust-to-weight ratios of approximately 7:1. This is still a much lower thrust-to-weight ratio than what is achievable with chemical rockets, which have thrust-to-weight ratios on the order of 70:1.