Nuclear thermal propulsion is controversial among designers because despite the improved thrust from the engine, the additional mass required by the engine and the rest of the payload for shielding and other things may result in a system that isn’t much better performing over a typical chemical engine.

An NTP engine uses stored liquid hydrogen, which must be very cold. The engine itself produces a lot of radiation, which heats and boils off the hydrogen before it can be used. So you have to isolate the systems with shielding. Then, depending on your mission, you need to worry about orientation to the sun and how to shield that well at the same time. It becomes a challenging thermal problem to extend the life of the LH2 long enough to the point where individual copper wires for sensors can conduct too much heat.

That all being said, NTP is seeing a comeback and systems that could be bound for Mars are under active development right now.