One of the anti-proliferation points is that U233 (which is the isotope you're thinking of) is mixed with U232, which decays to products with high gamma output, meaning that it's more dangerous to work with than the more typical fissionable materials, and much easier to detect. For a state this probably wouldn't be a significant barrier, but it's a fair point with respect to terrorist bombs. Of course, we haven't seen any of those, which implies that there's some non-obvious factor at work, here.