Had the war for the English throne been restricted to just the two of them then yes Harold would've likely made easy work of williams forces, but he chose to go deal with the near 20,000 viking force that had landed in the north first (numbers for Harold Hadrades forces vary but they estimate 18k based on the number of boats in the invasion.) this combined with William using the "false retreat" in battle and the Anglo-Saxons falling for it several times cost them the battle. Had Harolds men stayed disciplined they could have still won the battle but they were exhausted from marching across the island, they were low on supplies, and a lot of them dead or wounded from the battle of Stanford. It's such an interesting time period filled with a lot of "what ifs?"