So far there has never been a dirty bomb. What if the pro-nuclear fanboys get their wish and we solve the energy problems of the developing world using nuclear power? What if a modestly developed country gets overrun or overturned? I don't like the odds. Most risk calculations don't take into account that people seem to make mistakes or might eventually even blow up a reactor intentionally. It's unthinkable until it happens.
Also, same impossible calculations apply to any power tech. Number of people injured installing rooftop solar or wind, poisoned by graphite mining, trucking accidents, aluminum production, and the like.
And no, other power technologies are not as impossible to estimate. Nuclear power plants are very unique in the kind of mess they can make, see Fukushima and Tschernobyl. Those sites still need looking after and cleaning up, the final cost can't even be estimated yet. It's always been the accidents that supposedly can't happen and never happened. With other technologies not so much. Falling of a rooftop is a very knowable thing. Calculating the likelihood a nation state fails and terrorists get access to nuclear material for a dirty bomb? Never happened. But if nuclear power is your solution to the climate crisis, it needs to be everywhere...
And it's not "a major nuclear accident". Talk about multiple major and minor accidents on top of having a baseline of huge supporting infrastructure dealing with the procurement of radioactive base materials and the spent fuel and other waste. Because to replace fossil fuel and "solve" the climate crisis, you need to scale up nuclear power and its risks. Nuclear plants and their supply chains need to sprout up everywhere, including and especially where there is no trustworthy regulatory environment or where that is likely to disappear. And then we aren't just talking about accident but about potential mal intent which is impossible to calculate. Because once you build a power plant somewhere, you also can't just take it away.
Biological weapons are actually much harder. It does not just take "a lab". What are we talking about? Weaponizing anthrax? Unless you have access to the secret "weaponized" strains you'd need a pretty big lab to "breed" it yourself. At the very least animal testing, maybe human testing, definitely some mass production. That takes time. Also the dispersal methods aren't as simple as one might think. In theory bioweapons sound easy, in practice they haven't proven to be very effective.