Not to downgrade any particular person's handloading skills here, but reloading .357 SIG involves very careful sizing, seating and crimping to prevent bullet setback, does it not?. At the pressures it runs, there wouldn't seem to be much room for error before things go BLAMMO.
I, for instance, just don't trust my handloading skills enough to take on .357 SIG. I load .45, which is nice and forgiving for beginners like myself.