Obviously by making the final boss a female it implies that yuri is most difficult thing to overcome, literally for the player and metaphorically for the game dev. Practically speaking, it's a break from the rest. Tentacles, suckers, fingers are intrinsically phallic or tied to the male disposition. H-games will typical to the default 69 position or licking. A woman doesn't suck her own breast for pleasure, unless masochistic. While crafting a female adversary as a boss in an h-game it takes special care to not just make them a finger bang. Notably it should be expressed in less implicit ways. Thus far Hasumi and the Deep Sea Sisters have done this well.
Hasumi isn't directly attack by her known rival thus far. Subterfuge is the dark magical girl's main weapon, as seen by the newest game-over. When she faces Hasumi directly, it'll be her downfall.
