I don't relate to the various complaints about the multiple protagonist setup, but I think I intellectually understand it.
It doesn't work as well if if the player is locked into a particular viewpoint, such as some male players finding it difficult to relate to female characters, leading them to not want to play a female protagonist. Vice versa for some players with a male protagonist, although the female protagonist issue is more common because erotic VNs are a male-dominated hobby.
It doesn't work as well if some players might relate to one of the protagonists but otherwise doesn't care for what that protagonist is going through or trying to do. This happens for me with a lot of male protag games where the game is focused on something like trying to make a harem out of his family. I relate to being a horny guy, but don't care for that particular type of story.
It also doesn't work as well if the players put themselves really strongly in the shoes of one of the protagonists, and then switching protagonists becomes jarring. Maybe this is related to the immersion idea that Joe Steel raised. I tend to adopt the character's perspectives lightly, so I don't experience that jarring feeling, and when the day changes and the name changes from Ian to Lena or from Lena to Ian, I can mentally prep and switch. But I could see it being a problem otherwise.
I think multiple protag works best if you're flexible, can relate to both protagonists, are somewhat invested or interested in what happens to both of them, while not being too immersed in one perspective. When that happens, you can make choices for one protagonist while you as the player are more aware of the day-to-day context in which the other protagonist lives and what choices they're facing. It starts to feel like a real world with everyone facing complex challenges and circumstances, not just one person. I'm thinking of the scene at the bar where Ian notes that he heard from Lena that Louise and Stan don't get along, which is a superficial observation from Ian's perspective, but you (as Lena) have had actual interaction with that dislike and know how tense it is and how Lena struggles with that situation.