I love the Advantage/Disadvantage mechanic in D&D 5e. It's so simple and elegant, and it works perfectly with how the players interact with the d20 and the rest of the skills. In D&D, it's not about stacking advantages (in 5e anyway), it's about all of the other cool stuff the game provides. Adv/Dis is a easy solution that soaks up a wide range of situational buffs that D&D doesn't want to focus on.
So I borrowed it wholesale, implementing pretty much the exact same thing for Hostargo. I didn’t think about it again until after a playtest one day, and after a solid conversation with a good friend of mine, I got to thinking about “advantage” all over again.
But if you look at a game like Numenera, this simple solution actually doesn't work - at least not as well. This is because in Numenera, the "target number" to roll against is usually known to the player, and the player uses multiple different skills and abilities to determine if they can lower the TN (likewise, the GM would raise it for situational effects that make the task more difficult).
Known target numbers mean that it quickly becomes confusing whether the target number should change or if the player should have adv/dis. Hidden TNs on the other hand mean that the players only care about one thing (adv/dis) rather than two, and it's easier for the GM to apply adv/dis without debating back and forth with the players about the TN.
Skills and abilities must affect the players somehow, and in D&D they add to your roll as you try to get over the TN. In Numenera, they instead lower the TN. That brings the focus on raising and lowering the TN, rather than worrying about modifiers to the roll.
My game Hostargo is much more like Numenera: it has known target numbers, and as a roll-under system it was much easier to have skills/abilities affect the TN rather than subtracting from a roll [insert obligatory comment about THAC0]. The focus is different than D&D: I want the players to be thinking about ways to help each other out in order to boost their rolls. But if I only include an adv/dis system, that means that either A) the player has some ability that gives them adv, and no one’s help is required, or B) one person is able to help, and the rest are just chumps.
But what other options do I have? Abilities can directly affect the TN, but that’s crunchy (it’s much easier for the GM to have full control over that). Because I use dice sizes like an inverse Savage Worlds, I can have abilities grow or shrink die sizes, but that becomes confusing to new players on which die they should roll.
I don’t have an easy answer. For Hostargo, it just makes more sense to focus on the CR mechanic, and add or subtract from it. And that’s fine!