Also, RTGs have been used on earth, contrary to your implication they are only in space for safety reasons. Russian equipment in the arctic, for instance, has been powered by RTGs.
The point is you don't need huge complicated nuclear reactor to produce practically endless supply of energy in space, which is what the parent said he thought is a problem.
You probably need more energy than RTG but probably not as much as most people think. If you can apply a steady force for years, you would be able to give the required 10-100cm/s velocity delta to even large asteroid.
There are and have been fission reactors in space already. RTGs seem to typically be in the hundreds of watts, while the reactors are more in the kilowatt range.
The reactors are not RTGs, and they are definitely a lot more complex.
But really, reactors are extremely simple. For example, on Voyager missions the it is just a hunk of metal that is constantly hot.
https://en.wikipedia.org/wiki/MHW-RTG
You then do with that energy whatever you want.
You can use part of it to power electronics and actuators, part to melt a piece of asteroid and part of it to convert it to ions and send whichever way you want.