Uneducated Silver Player, Here, Requesting Details On An Item

Okay, I seriously need some answers and details and all that good stuff regarding a certain item within League of Legends. The item in question is:

[Executioner's Calling] And while we're at it, let's just include it's final form, [Mortal Reminder] for clarity and stuff.

Anyway, I often have games where the enemy team has, like say, a or a or a or hell, maybe even an . And while playing some of these games, if any of the aforementioned champs got like REALLY far ahead, like I'm talking snowballing out of control, I'd get that one teammate who says something along the lines of "they're too fed. we can't even kill them because they'll just heal back up during the fight." Also note that this is usually an AD champion on my team who is saying this.

Now personally, I would just finish my first item or two and then build the and then finish off the once I got some other Armor Pen item, so that I could take advantage of the bonus Armor Pen from Mortal Reminder's passive. After building the item, I feel much safer fighting said champ. Building this item, however, doesn't ensure my victory against said champ. It just seems to lessen the amount of stress I'd have to endure fighting a Champion who is constantly healing during a fight/skirmish.

All that aside, my teammates never seem to buy it. Like, literally no one I've ever been in a game with, for the past month or so even considers building it. When I try recommending it to the teammate from the earlier example, I get one of two responses:

Either (A) They buy it with no questions asked. Which only happens like once every blue moon. Or (B) They choose not to buy it, flame me for even speaking of the item, or both. Either way, with this option chosen, that teammate continues to complain everytime they get killed by whatever one of those champions they are fighting.

Is there something I'm not seeing here about Mortal Reminder? Is it not doing what it was intended to do, halt/stall healing? Is it just not as strong as it should be?

I will admit that I have no idea how to do the whole "damage calculator" math thing like most other Summoners, here, so I would like for someone to help me out here. Even if you have to bring up another item for comparison purposes. I just need answers! Please and Thank You!