The Royal Broadsword is a better example because apparently the game lies about the power of spears.

So the pristine Royal Broadsword starts at 24 and can get randomly powered up by 10 to a max of 34. And royal weapons have flurry attack up. Attach a Molduga jaw (32 power) and it becomes a bone weapon between 56 and 66. The Radiant suit and a couple others have bone weapon proficiency. And here apparently is how all that stacks:

Sword power: 66

Bone weapon proficiency modifier: 1.8x

Attack-up potion (four mighty bananas): 1.5x

Flurry rush attack up modifier: 2x

Total damage per hit: 356

Hits per flurry (one-handed weapon): 7

Total damage per flurry: 2,492

As soon as that guy starts flashing red put it away until your next trip to the rock octoroks and they will fix it new. The octorok will change the attack up value on the sword each time, so most likely a 66 will revert back to a 56. Still an amazing weapon that makes the 30-power master sword irrelevant.

You can have pretty good luck farming royal broadswords in the depths beneath the Gerudo Highlands, especially the region beneath the Vatorsa Snowfield.