They have a lot of advantages, but they are extraordinarily expensive, screamingly radioactive, require nuclear weapons technology to manufacture, and are ineff

They have a lot of advantages, but they are extraordinarily expensive, screamingly radioactive, require nuclear weapons technology to manufacture, and are inefficient in the amount of energy you put in vs. get out. Even for space missions, the tradeoff of just using solar panels (which have a ton of problems in space) is often worth it. It's not easy to get an RTG approved for your multi-billion dollar space mission.