Quickleaf
Legend
I really love the elegance of the +/-5, at least in concept, and even have my own fantasy heartbreaker built around it.weapon masteries that have a save can be simplified with +/- 5 mechanics.
Instead of save vs. prone, if you beat AC by 5 or more, your target is prone.
But I've really struggled getting it to work in my D&D games.
My players usually don't keep track of monster AC, even when I try different tricks to display it, it just slips through the cracks. So getting them to the next step of "did I hit by 5 above its AC" was next to impossible and involved several exchanges between GM and player each round.
Then when I would try to track "did they hit by +/-5" across multiple PCs with multiple attacks against enemies with varying AC... and in 5e at least it became a mentally exhausting for me as GM to handle smoothly & efficiently. I feel like with 5e there's enough mental burden, this added burden doesn't work for me personally.
But in a lighter / OSR system? I think it could awesome.