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I think both isekai and battle harem are able to hit their respective forms of wish fulfillment if executed half-competently, but I think the difference lies in their scopes.
>battle harem
Has a clear end point given the current world state. Your chicks are going to beat the shit out of you, and have sex, or beat the shit out of each other, then have sex with you, or work together to beat the shit out of an even bigger girl, then have sex.
It's designed to be self-contained and not leave the viewer longing for much more, and you don't need to expand a lot other than how powers work.
>Isekai
The plot is a lot more open-ended compared to battle harem. The point of the isekai is that it's a vast and ever rotating world, which leaves a lot more potential for overarching story telling. The world state is also used as a central moving point in most isekais, mostly in the form of how the self-insert has been slighted by how things are currently, or how they are actively trying to change them. A goal that big can't be done in a single season, which is why most writers seem to opt to just giving the self-insert some bullshit power or have them crutch on their modern past life experience to fast track them to either living in comfort, or blasting away the opposition. This type of mentality also affects the worldbuilding, since the market is that people just care about being literally them in a fantasy world and reaping all the rewards, so the world outside of a few snippets in the start, and some plot twist later, is almost deemed irrelevant, since the point would be that it will be forcibly changed by the MC.
Tldr: The scope of isekai is much larger than that of a battle harem, so a viewer might feel bad over the wasted potential an isekai can have. Both can do similar things, but the battle harem is on average better about owning up to the schlock factor and just having fun with it.
