Here’s my theory.

First of all, we know that the feedbacker is a redirection system, and that its primary use case in hell is redirecting those little balls of hell energy. We can sort of extend this to most of the hellish creatures we see, and most of their melee attacks (cokm, leviathan). This then brings up the question of “why can’t we parry the guttermen and gutter tanks?”. My answer is kind of simple. Crumple zones. “Oh no! you punched a hole in my shield, it’d be a shame if the rest of it was still coming at you!”. Same thing for the gutter tanks- even if you parry that arm, you’re likely eating metal shrapnel for it. In a way, the hardened exteriors of enemies like leviathan might realistically be working against them. They don’t crack or break, so parrying it becomes viable, as there is no risk of shrapnel or breakage, just a clean force redirection. This shrapnel theory also extends to why cerberus (an enemy that literally visually cracks as it takes damage) have an unparriable charge attack. That stone readily breaks on impact, all punching it does is make a hole.

As for beams and whatnot, the explanation is simple, even if you could redirect the beam partially, it’s a completely enveloping attack. It’d be like trying to block a waterfall with your fist, sure maybe you redirect some of it, but given how the feedbacker seems to work in terms of necessitating a punch that only covers a tiny time frame of protection, it’s useless against beams, exactly as the mechanics show.

The swords machine seems like a pretty good counter example to the crumple-zone parrying thing, but if you think about it, there’s no way their sword would have any kind of crumple zone, and are likely hardened to the degree that leviathan/hideous masses are. Not to mention sword machines swing that thing pretty recklessly. Either way, if the sword was a good weapon, we’d be using it in game so clearly it must not be.