Basically, none that we need to be concerned with. The general idea is this: when you fuse elements, you produce heavier elements; if you wanted to fuse these

Basically, none that we need to be concerned with. The general idea is this: when you fuse elements, you produce heavier elements; if you wanted to fuse these new elements into even heavier ones, you'd need to input much more energy — thus it can be said in layman terms that the products of fusion are 'toxic' to the fusion itself, i.e. the more you do it the less likely it is to go on, until if finally stops (like in real-world stars). For instance, the Helium produced by the Sun is making it harder to sustain the Hydrogen fusion, which will stop before all Hydrogen is depleted. In addition, the radioactive elements produced by fusion are much lesser than fusion, by orders of magnitude, so it's pretty much 'clean' in that regard for all intents and purposes (50 years of fission probably produced much worse than 1,000 years of fusion would). And the fuel is available in pretty much limitless quantity — in fact, it would probably make a space industry, starting with mining hydrogen isotopes for fusion, profitable from day 1. It's basically the closest thing we have to God's tech, and there's not much else in theoretical physics that comes close in terms of benefits in our real-world context. Fusion would essentially hand over the galaxy to us with little to no chance of failure, given enough time. If we can pull it off, that is. Which remains to be seen, there's nothing new or groundbreaking in this article.