Embrittlement and neutron activation are already well understood and perfectly tractable problems, since they affect the fission power industry to the same degree. That comes down to the materials you use to construct your reactor.
There are thre main problems, only one is entirely engineering the other are on the border of engineering and physics. The most pressing problem is one of plasma containment lifetime. The longer you can contain a plasma the more feasible it is to operate a fusion reactor as a power source. Right now that time is measured in seconds. An equally important problem is plasma temperature, the higher the plasma temperature the higher the fusion rate, and the farther beyond break-even the fusion reactions will be. ITER is "designed" for containment times of up to a thousand seconds, about 15 minutes, though in reality it is likely to fall far short of that. And even that is a far cry from what would make fusion power production feasible engineering wise. As it happens, tokamaks are not necessarily particularly well suited to long containment times, but there hasn't been sufficient funding to thoroughly research all of the most promising designs (such as stellarators, gas-dynamic mirror fusion, spheromaks, etc.). It seems extraordinarily unlikely that the first and most easily constructed plasma confinement system would be the most capable one as well.
The main purely engineering hurdle is cost effectiveness. Which is about being able to construct a plasma containment and heating system and all of the other components of a fusion reactor (some straightforward, some not) within a reasonable cost, not multiple billions of dollars for a single 1 GW plant. There breakthroughs in superconducting wire and so forth can have an enormous impact on the economic viability of fusion power, so it's a hugely important step.
There are thre main problems, only one is entirely engineering the other are on the border of engineering and physics. The most pressing problem is one of plasma containment lifetime. The longer you can contain a plasma the more feasible it is to operate a fusion reactor as a power source. Right now that time is measured in seconds. An equally important problem is plasma temperature, the higher the plasma temperature the higher the fusion rate, and the farther beyond break-even the fusion reactions will be. ITER is "designed" for containment times of up to a thousand seconds, about 15 minutes, though in reality it is likely to fall far short of that. And even that is a far cry from what would make fusion power production feasible engineering wise. As it happens, tokamaks are not necessarily particularly well suited to long containment times, but there hasn't been sufficient funding to thoroughly research all of the most promising designs (such as stellarators, gas-dynamic mirror fusion, spheromaks, etc.). It seems extraordinarily unlikely that the first and most easily constructed plasma confinement system would be the most capable one as well.
The main purely engineering hurdle is cost effectiveness. Which is about being able to construct a plasma containment and heating system and all of the other components of a fusion reactor (some straightforward, some not) within a reasonable cost, not multiple billions of dollars for a single 1 GW plant. There breakthroughs in superconducting wire and so forth can have an enormous impact on the economic viability of fusion power, so it's a hugely important step.