the ax1200I should be able to run the neptune without any special bootup procedures right?
Based on what Neptune users have reported thus far, you WILL need special bootup procedures for this PSU. See KnC's troubleshooting guide under the heading "PSU powers off when ASIC boards are connected".
If you haven't bought your PSU yet, get EVGA 1300W Supernova G2, good reports so far. Or Seasonic X-1250.
Im not an expert but maybe it has to do with initial power draw from the asic boards to the psu. And every psu is not the same how they handle this initial load, which is different than a continuous load draw after everything is running. It takes more load/torque/power etc to start something than to keep it going.