D-He3 (deuterium and synthetic helium) is attractive, except you have to make the He3 by making tritium and waiting for it to decay. pB (hydrogen and boron) is attractive, but much harder to make work than D-He3.
There is work on a D-He3 rocket, tagged "Princeton FRC", that they hope to loft for a trial maybe in 2035. Elon could get it tried out earlier, and ought to. We should be cranking tritium production up immediately so there will be enough He3 when we need that.
DT fusion reactors would be very heavy. ARC, for example, would have the weight of a couple of US WW2 destroyers. DT fusion delivers its energy as heat. A fission reactor would be far smaller, simpler, cheaper, and more reliable.
You might make the case for advanced fuels, but that likely means you have to get 3He from somewhere. I suspect beamed power will beat even advanced fuel fusion in all missions in the future (a weak prediction as I will not be around to see.)
In space, the fission doesn't need containment, or even much shielding if it is far enough away from the rest of the ship.
In the cloud tops of Titan, Venus, Saturn, Uranus, or Neptune, a nuclear power plant may be as simple as a naked pile suspended near the bottom of a big fabric tube with wind turbines at the top. The radioactively-heated, buoyant air inside holds it up, and, escaping through the turbine, generates power. (Enough is held back so it stays up.)
Perhaps surprisingly, gravity is almost Earth-normal on all four planets. Perhaps more surprisingly, the temperature at a not unreasonably-high pressure is also close to Earth-normal (except on Titan). But getting back to orbit from any of the last three would be very, very difficult, so such a reactor would only ever power robots.
Getting back to orbit from Titan is easy. From Venus, it is pretty hard; and once in orbit you would need to refuel to get anywhere else.
Fusion has all of the characteristics of something you'll want to do if you are in space. Add to that the fact that its fuel is much easier to acquire than fission and that solar isn't available everywhere and you have a winner.
Meanwhile down to Earth, yes, most likely it will never be an important energy source.