The most interesting possibility here is nuclear alchemy. If the practitioner can have very fine grained control over the weak force on bulk materials, the possibility exists that they could trigger specific modes of radioactive decay in specific elements, causing them to become other, possibly more useful, (and perhaps even stable) elements. "Impossible" materials which would be too short lived in real life could be made stable, at least whilst under control of the practitioner. This also means that you could, in fact, transmute lead to gold, assuming you were adequately protected against the interesting varities of radiation that the intermediate states would produce. You wouldn't be able to create heavier elements this way... no transmuting lead into uranium, as gluing extra nucleons on needs the strong force (but if you have a wizard who can control that, maybe you could work together). Only transformations that conserve or reduce mass number would be allowed.
Note also that the weak force allows for the transmutation of a proton or neutron into its own antiparticle. This isn't elemental or isotopic transmutation, and as the resulting and inevitable annihilation with any nearby nucleons is not mediated by the weak force you'll get a load of quite unpleasant radiation in pretty short order. If you're not mkaing antihydrogen for CERN, this might only be useful as a sort of suicide bomb.
The practitioner would not be immune to the effects of radiation, as most damaging radioactive effects are mediated by electromagnetic interaction of ions or short-wavelength photons with matter. They may on the other hand be able to suppress radioactive decay in their body or immediate surroundings, making them much more resistant to the effects of radioactive fallout and you certainly wouldn't be able to pull a Litvinenko on them. They'd be able to benefit from cheap land prices around Fukushima, though they might not have many guests.
Given that depleted uranium is seen as something of a waste product of a nuclear-armed power's enrichment programs, turning this hazardous waste into valuable material might even be seen as something of a public service, and has the added benefit that uranium is so heavy that you could create almost any other useful material from it. Similarly, denaturing radioactive waste into shorter lived isotopes or even stable ones would be very valuable indeed.
Depending on how you want your powers to work, people with this ability might have some kind of material sense... you can't work out a nice decay path if you don't already know what collections of protons and neutrons you're dealing with, after all. How that turns into a useful power is up to you, but you can be sure that an alchemist knows their lead from their gold, and you'll not be able to sneak debased currency past them.
Killing people or damaging things by making them intensely and briefly radioactive or spontaneously fission seems a little crude in comparison.
Power-balancing this seems likely to be awkward, though restricting the range of the effect might be thematically appropriate. Don't accept massages from people wearing lead aprons and gauntlets, in case it wasn't already suspicious enough! Restricting the effects to elementally or isotopically pure materials might also work, so you might transmute a lead pipe but not a person.
The biggest problem is of course what to call them. "Weakling" would be an obvious epithet, especially from people capable to chucking lightning bolts or levitating, but probably best used only by those who are bored with the lack of cancer in their life.