The problem with sword combat isn't the computational complexity, but the fact that reporting a hit late is much more noticeable.
If you are running across some open space and a bullet hits you, the game can report you being hit 500ms later and it's perfectly fine. With sword, the player can actually see the sword hitting and it has to report the damage at the same moment.
A game can compensate by having slower strike animations and preventing players to redirect sword mid hit. Dark Souls 3 does a pretty decent job there - once you press the attack button, you commit to a the attack animation and cannot change or cancel it. So, the receiving end can speed up the attack animation somewhat to compensate and show the sword hit approximately at the same time it hits "on the server". As long as you have good ping, the game feels close to real time - except that opponent attack animations feel somewhat faster than your own (so, harder to dodge and parry).
If you are running across some open space and a bullet hits you, the game can report you being hit 500ms later and it's perfectly fine. With sword, the player can actually see the sword hitting and it has to report the damage at the same moment.
A game can compensate by having slower strike animations and preventing players to redirect sword mid hit. Dark Souls 3 does a pretty decent job there - once you press the attack button, you commit to a the attack animation and cannot change or cancel it. So, the receiving end can speed up the attack animation somewhat to compensate and show the sword hit approximately at the same time it hits "on the server". As long as you have good ping, the game feels close to real time - except that opponent attack animations feel somewhat faster than your own (so, harder to dodge and parry).