They are the worst.
They are annoying failures in every game, and I don't want to see DSRPG3 make this mistake. Slayers shouldn't need this life saving buff that exists for no reason.
The only slayer marks in this game should be COSMETIC. Maybe you can buy a slayer mark that cools COOL, but does NOTHING. That's something I could get behind. Other than that, they're a really dull, useless concept that doesn]t fit into the game, truthfully.