You can make a centrifugal magnetic mirror the size of a truck for a few million bucks that will weapons grade as much fissile material as you want. It doesn't take new science or technology to do. If you want to keep weapons grade fissile material out of someone's hands then your best hope is preventing them from having fissile material (or at least controlling every pound of the stuff).
Why is Iran running hundreds of centrifuges if they could cheaply use one of these things to make it into plutonium and separate that out by trivial chemical processes?
Maybe because it's the US cold war blueprint and doesn't require any innovation. To be clear: no public program has used magnetic mirrors for enrichment but they are so cheap/small and so hot that they really are a sure thing. Maybe MRI magnets are difficult to source. I feel like the true answer is not rooted in any technical explanation but rather some kind of political explanation.
If I was a warlord trying to make industrial quantities of weapons grade fissile material in 2022 I personally wouldn't go the centrifuge route, but everyone's a critic.
The most intense non-fission, non-fusion neutron source in the world in 2017, Oak Ridge Spallation Neutron Source, produced ~1e10 neutrons / second. If every last neutron emitted could be cooled and made to be absorbed by a ready U-238 nucleus, decaying after a few days to Pu-239, if I figure right we ought to get a microgram in 28 days, or a Kg in 78My. Pu-239's half-life is only 24Ky, decaying to (equally useful) U-235 with half-life a solid 0.7Gy, so only a little bit turns to lead before we finish.
Keeping our copy of the Oak Ridge source operating, and the current world order destabilized, for those 78My seems difficult; and we anyway have made only a Kg. So, this seems like not a practical way to generate a geopolitically effective amount of Pu-239 or U-235.
Magnetic mirrors are fusion devices, but they are pulsed and have conduction losses due to the central electrode. This electrode greatly increases confinement performance but is obviously an impossibility for power generation. Where magnetic mirrors really thrive is in having very hot plasma in relatively small devices [0].
So the idea is not to use magnetic mirrors for power generation, but as a DD-fusion-powered cheap fast neutron source. I have a hunch that radioisotope companies and government organizations are doing this right now.
