If players are having a difficult time finishing games I would look to how much difficulty scales throughout a single game.
Oh, what do you know- scrakes and fleshpounds only spawn in the second half of the game, plus there's a boss after all the waves. I wonder if players might be having a difficult time with them?
Additionally, if we're altering balance dynamically, what does that mean for the first half of the game that's relatively easy compared to the second half? If I'm kicking *** against clots, crawlers, stalkers, bloats, gorefasts, sirens, and husks I sure as hell don't want that performance to increase the difficulty of scrakes and fleshpounds (in addition to all of the previous zeds) once they show up.
As things currently stand, I think scrakes, fleshpounds, and bosses could be eased up a bit on lower difficulty levels. Level out the difficulty a bit from first wave to last of a single game but let it jump more between difficulties (norm, hard, sui, hoe). After getting used to the final wave of one difficulty the first wave of the next difficulty should not be too much easier.