The primary advantage of early-generation NTRs is that they can operate with pure hydrogen, that's it. Doing that allows them to have an exhaust velocity of around 9 km/s. And because the rocket equation is exponential with respect to the ratio of delta-V and exhaust velocity NTRs start to look really good for single digit or low double digit delta-V. With a stage mass ratio of 5:1 you can achieve a delta-V of 14.5 km/s, which is a lot to work with. In contrast, with the same stage mass ratio you'd achieve maybe 40% of that delta-V with a LOX/methane stage.

However, things stop looking so rosy very rapidly. Because NTRs use a heavy reactor and rely on low-density hydrogen it is very challenging to achieve high stage mass ratios, which limits performance. Also, because liquid hydrogen is super cryogenic and has a high boil-off rate it is very challenging to build a high efficiency NTR which has significant longevity for deep space propulsion. Even if you can bring boiloff rates under control with thermal control systems and active cooling all of that stuff adds mass which again cuts into the stage mass ratio.

All of which conspires to make the most compelling use of a first generation NTR something like a trans-lunar (or interplanetary) kick stage for crewed missions. Which might be fine, but is still pretty limiting, and likely results in only a small number of NTRs ever being built.