Aside from pressure considerations (shouldn't be that much of an issue with extremely light loads, I would think), I would be more concerned about slam fires in semi-autos with floating firing pins. Of course, extremely light loads won't cycle such guns anyway, right? But how about the act of dropping the BCG of an AR on a pre-chambered light load when single feeding it? Would the firing pin have enough inertia to detonate a thin SP primer? Dunno. Spitballing here.
In larger cartridges, in single shot rifles, I use LP primers in lieu of LR primers with zero side effects. Specifically .32-40 cast target loads comprised of 14 grains 4227. Possibly comparing apples to oranges here though.
