I think the "promise" of fusion is a huge misunderstanding.

I mean we already have a nearly infinite source of clean energy, it's called fission. The only difference between fission and fusion is that fusion will be much more expensive.

If you want a truly unlimited source of energy, we have those too, they're wind and solar.

Seriously though, the expectations for fusion are completely rediculous, when we finally do get it working, it will be the most expensive form of energy ever concieved. If the reactors use the standard method of generating energy, heat capture to run a turbine, it will also require enormous amounts of beryllium as part of the "blanket" around the reactor. How much beryllium will be needed? In the whole world, we probably have enough beryllium for 4 grid scale reactors, the cost of which would be astronomical.

Here's the worst part, over time those blankets would absorb neutrons, the materials would degrade and eventually the now radioactive blanket would have to be disposed of and replaced.

The tldr is this - tokamak and stellerator style fusion reactors work great in theory, they will probably successfully make sustainable fusion reactions quite soon. But they may never generate electricity in practice, they're a logistical and economical nightmare.

I do think fusion could make a fantastic spacecraft engine however, I expect that will be a huge application.