A Pu bomb is much harder to blow up than a U235 bomb. The tolerances for the sphere must be very precise, else you get a fizzle, which is what happened to the NoKo test.
Iran is using Uranium. Unlike Pu, the shape of the U235 doesn’t really matter once it reaches critical mass. A simple rail-gun will set it off.
The US nor the USSR ever dared to put U235 into an ICBM missile. It was too dangerous. The G-forces during launch could set it off.
By the late 1950s the US and USSR abandoned U235 bombs entirely because they were too dangerous to handle.
But if you’re a mullah who doesn’t dare about the occassional ‘work accident’, well..
It's easier than that: with the right casing (proper amount of beryllium and heavy enough to contain the initial neutron storm), all you have to do is use an explosive to plop one sub-critical chunk of uranium into another bigger chunk, that becomes critical when "assembled".
Remember, in WWII, "Fat Man" (a gun-assembly uranium bomb) was considered to be so reliable it wasn't even tested before being dropped on Japan.
“By the late 1950s the US and USSR abandoned U235 bombs entirely because they were too dangerous to handle”
I don’t know the answer on the “danger” of U235 warheads, but you are in error about them being abandoned in the 50s. The military still had canon fired tactical nukes in the 80s. Army field artillery units (155mm & 203mm) routinely trained for using them.
That U235 is easy to get in a supercritical state makes it ideal for a nuclear newcomer.
You are correct that Pu239 weapons design is a much more challenging thing accomplish. The first atomic bomb (a Pu239 weapon) (Trinity) that was tested at Almagordo, NM was done so because they weren’t entirely sure it would work. The first U235 weapon was tested by dropping it on a Japanese city...because they were certain it would work.