People primarily base their expectations off of past experiences. Guro is rather niche, so the vast majority of h-games don't have it, regardless of their tone or themes. Even in normal video games, it's not uncommon at all to see games with a "dark tone" that lack particularly graphic violence, such as dismemberment or disembowelment. Yes, maybe it could fit into a dark setting, but that doesn't necessarily mean you'll always find it there, or that you should always expect it. Additionally, Night of Revenge went quite a long time without any guro elements at all, so that would also influence people's expectations, I'd imagine.
I'm not against guro, but I think the implementation leaves something to be desired. Not only are the scenes spread across the game in a seemingly arbitrary fashion, but they tend to be pretty dry as well, as if pointlessly straddling a line between trying to appeal to people who like guro, and trying to avoid alienating people who don't. The result is a handful of scenes that don't seem like they would fully satisfy either party. Furthermore, there's also a concern that enemies (or bosses) that get a guro scene won't get a sex scene, so it feels like the guro scenes deprived players of something better. They're shaping up to be a rather weak element, added in too late to really get the development time and consideration they need.
I don't think people disappointed with the guro are being unreasonable, or that they had unreasonable expectations. I also don't agree with the idea that 'dark themes' is just a single nebulous mass of ideas and concepts and that where you find one 'dark thing' you should always expect to find all the other, vaguely-related 'dark things'. I believe this manner of thinking does Miyazaki's settings and motifs a particular disservice, since the thought he puts into crafting them has more nuance than that.
On a less stuffy not, I have to say that the idea that the guro that should have been in Eluku's games got cosmically displaced to Night of Revenge does seem amusing to me.