[VLM]

"one of its safety features compared to fission"

Not exactly. Off the top of my head I can't think of a reactor thats unstable in operation. Hands off (like that TV series life after humans where the people all disappear) almost all stabilize and eventually shutdown on their own. Maybe those crazy graphite reactors are not inherently stable.

There's two linked issues.

Fission reactors get about 10% of their heat from decay products (the "waste") decaying away. That means there is no instant off switch. The "off" position is still 10% out for hours/days/weeks (well it decays away eventually...). As the Japanese found out the hard way, 10% of a huge amount of power is enough residual heat to cause an awful disaster.

The other is surface area / volume ratio. Couple gigawatts thermal like the Japanese and there's not enough surface area to cool it without giant pumps when shutdown. Couple MW like a nuclear sub and the surface area ratio is better, theoretically you could probably walk away from one without anything awful happening. The people who know aren't talking. Fusion you're talking about something a millimeter across not multiple meters. Walk away or have a computer crash or whatever and it seems impossible for something that small to cause much damage.

There are some practical physics reasons why making a fusion reactor the size of the sun is really easy and the smaller you go the harder it gets, but there are practical engineering reasons why making one bigger than a millimeter would be a huge PITA. On the other hand if you could make a fission reactor the size of a millimeter that would certainly solve a lot of painful thermal engineering problems, but the physics just doesn't work (long story...)