Gotta hand it to him - possibly one of the unluckiest highly-competent warrior kings ever. Could Alexander have won two major battles within weeks of each other, on the basis of part-time militia? Harold certainly failed.
He would likely have prevailed if he'd waited for full mustering in London before heading to the landing site. But there are bound to be things we don't know, such as, William had turned some of the vassal lords to his side. Only a handful of them retained their titles and property after the regime change, most of the rest died doing their duty on the battlefield.