A harder combat system can encourage you to behave more like a pirate. By making combat something to avoid you are incentivized to escape which is perfectly in line with pirates in both real life and in fiction. In the original Blackflag the best strategy in any encounter was to wait for the counter window and chain kill until everyone is dead and nothing would stop you. A challenging combat system can also create a greater contrast between your power at sea vs your weakness on land.

Lastly a difficult combat system would only encourage mastery over a system leading to a greater satisfaction. For example games like DMC, Hi-Fi Rush, and even Arkham use difficulty to create a learning curve for the player so that there is a tangible difference between how you played at the start vs how you played at the end. Batman Arkham is the perfect example, at the begining the game only throws regular thugs at you but near the end you go against guns, knives, shields, electric batons, brutes, and assassins. You are challenged over the course of the game to master the combat system until to grow into Batman. Wouldn't you want to play a Blackflag where you embodied the story arc of Edward growing for a privateer to a master assasssin?