At the risk of getting off-topic, part of this is probably due to the story setup. The party is never "split" for a long journey in BTW unlike in Nephilim, so there really isn't a scenario where MC and Hannah would be separated for a long period. As for the harem route, the other LIs definitely start out casually, but it's rapidly becoming more serious polyamory as the relationships with the other LIs grows (which makes sense, given Hannah has a two-year head start).
That does touch on one of my biggest gripes with the Nephilim though. Namely, that on the poly/harem route, the LIs pretty much don't acknowledge the other relationships. I prefer there to be more discussion about things like mutual consent when it comes to including multiple partners, which is something BTW does very well. This I think also contributes to the complaint you mention, since if there's not collective discussion between the LIs about multiple relationships, it makes them all feel very separated and the game can only focus on one of them at a time to the exclusion of others.