Many tanky champions are still sustain reliant, such as Dr. Mundo and Trundle. Mortal Reminder may be a better general purchase against those champions, since it applies Grievous Wounds during teamfights, allowing your entire team to combat them.
Since Mortal Reminder no longer provides armor penetration as of V10.23, it is a much better choice against champions that are sustain reliant but don't stack a lot of armor, such as Vladimir, Akali, and marksmen with lifesteal items such as The Bloodthirster.
Unique Passive - Sepsis: Dealing physical damage to enemy champions inflicts them with 40% Grievous Wounds for 2 seconds. Attacking a champion 3 times within 2 seconds increases this effect to 60% until it ends.