Production and storage remain a hurdle. Not quite describing how grotesc that hurdle is!
Lets see:
-10nanograms a year means it would take 100.000.000 years to make a whole gram.
-Right now you cant even contain a simple fission reaction for a practical engine, you can still only dream of containing a fusion reaction on a terrestrial energy plant. Containing a matter/antimatter reaction is just unimaginable!