Don't believe the myth that a matter/antimatter blast converts 100% to destructive force. You lose about half the blast energy to neutrinos. (If the neutrinos were crazy dense enough, they could cause breakdown of nearby fission weapons, hilariously enough - rendering nearby nukes inoperable.)
I don't see neutrinos involved, really.
Electron-positron annihilation produces only gamma photons of about 511 keV energy (or more if the particles had lots of momentum before their collision).
Proton-antiproton annihilation (at lower than about 2 GeV energies) produces four gamma photons and a chargeless pion, which decays into two gamma photons.
Neutron-antineutron annihilation produces two gamma photons directly.
What you lose in destructive force is the fact that gamma rays are stupidly penetrating stuff. Most of the gamma radiation passing through stuff like human tissue just goes through doing nothing, which means you need quite large intensities of gamma radiation to actually cause a harmful amount of ionizing impacts on the tissue (this level depends on the tissue, some are more sensitive to damage than others). Additionally, the intensity (of course) reduces fast as distance grows, in the inverse square of distance.
Hence, despite the massive amounts of raw energy unleashed, harnessing it into mechanical work is not quite straightforward. The gamma radiation itself doesn't cause immediate damage unless it is so dense that it transfers huge amounts of energy into nearby materials and vaporises it, but due to the penetrating properties of gamma radiation this would require quite a massive yield, and most of the energy would still escape.
The best way to actually utilize the energy of gamma burst released from the annihilation is to encase it in a non-solid fragmentation shell made of lead or other very dense element (but lead is probably the cheapest, depleted uranium tends to be a bit sparse) which would attenuate the gamma radiation significantly, heat up by a corresponding energy, and turn into expanding cloud of lead vapour and parts of the outer fragmentation shell. This could cause more damage in the vacuum.
In atmosphere, similar system could increase the amount of energy captured at the blast site. If that were not done, the annihilation would produce a mass of gamma rays which would probably heat up the air enough to cause an impressive fireball and lots of radiation sickness nearby, but if there was a shroud around the annihilation that would gather the majority of the gamma rays released, it would constrict the energy release to a much smaller area, and would likely result in pretty much larger destructive power overall.
EDIT: Typio