What conceivable risk could there be that supports this hypothesis?
For one, fusion reactions generate neutrons that activate all the structural material of the reactor, creating radioactive waste. The waste is shorter lived than fission waste but is still a risk.
A more obvious risk is the pure destructive power of that much energy. High pressure, 100 million degree plasma is full of risks. It's nice that fusion reactors can't "run away" like fission does, but you still have to deal with material 6x hotter than the sun escaping confinement.
The specific heat of Hydrogen gas is roughly 20 J/gC at high temperatures[1]. It would be much higher for a plasma but I couldn't easily find a number.
At 100M degrees C, a single gram of hydrogen would have over 2 Gigajoules of thermal energy. 2 grams of hydrogen would have the same energy as a literal ton of TNT.
It's not untenable, but its not really something that should be brushed off as "its only a couple grams of fuel" or "it can't have a runaway reaction".
That plasma is at less than atmospheric pressure, and is contained in a massive steel vessel. Even if it expands and melts through the vessel, the atmospheric pressure will collapse it. I do not see a plausible scenario where anyone is in jured.