1. The game is admittedly a structural mess; the story is presented in non-chronological order on a scene to scene basis so you never know when on the timeline any given event is taking place and you're left to put the puzzle pieces together yourself, since it's never straightened out for you. At all. There are clues, but you simply don't have the information to put any of it together until I believe the last scene in the game so it's something that has to be done retroactively. Personally, it took me weeks to get a decent understanding of what took place. It also helps that the plot in this game is by far the craziest thing I've seen in eroge thus far, good god.

2. There are links in the text that lead you to in-game encyclopedia entries on a myriad of subjects ranging from Erik Satie's life to neuroscience to explanations on how deadly a crossbow actually is when used as a murder weapon to glasses to etc etc etc.  Inside of those links, there are even MORE links that lead you to different encyclopedia entries, so you can end up 5 or more levels deep at times. Many of these are so long, packed with information, and unrelated to the root scene that they completely break the flow of an already extremely confusing story and require you to use an exorbitant amount of brainpower to keep up. A lot of these are very interesting but there's no denying that it is information overload and this makes it extremely difficult to stay focused. On at least three occasions I completely forgot what the original scene I was on was even about when I made it out of the encyclopedia entries, to give you an idea of how long they are. These end up creating a kind of "sub-scenario" I guess and they actually do end up being relevant to the story or used in interesting ways, so I don't recommend skipping any of them.

3. Unlike Romeo's other work with the exception of Natsuyume Yawa (surprisingly enough), he uses an EXTREMELY grandiloquent and complex writing style here. People coming straight from C+C or Kazokei will probably be very surprised, the game is quite difficult to read in places and I have no problem admitting I had a great deal of trouble with it throughout.

4. The subject matter the game is dealing with is mostly "high concept" stuff that was mostly alien to me personally, so I had to spend a lot of time looking up Jungian archetypes and various kinds of philosophy and things of that nature. I don't pretend to have an understanding on everything it talked about, but boy there's a lot of very complicated stuff it's playing with. For what it's worth, Ima definitely taught me the most out of any eroge and I felt that it used it all in an amazing way, it's really quite something now that I'm thinking about it. I remember saying when I finished I felt like I just had brain surgery and I still think that's a pretty accurate description.

I could go on for several more points on things I found difficult but I think that should be more than enough to show why I consider it to be the "hardest eroge" out there. I don't really partake in eroge difficulty olympics and I hate dumbass """difficulty tier lists""", but Ima really does require a high level of reading comprehension to even be worth your time. I mean, if you're constantly questioning your own understanding of Japanese on top of everything else the game does, how can you possibly hope to make it through this? I read it some 60 or so games in, fairly confident in my Japanese ability (lol), and also went in with what I felt was an appropriate gravity for the game and still got completely mindfucked by it. I won't tell anybody not to read it, but it's probably a good idea to be aware of what you're walking into.

PS: I'm sure the game sounds like a mess based on this answer and I won't deny that it kind of *is*. Saihate no Ima is unconventional and obtuse and abstract and HARD to understand, but don't let that turn you off to it. I find what it did for me to be exceedingly difficult to put into words, even when talking with people who have finished the game, but it's one of those things that I think everyone should go through. In the end, I honestly couldn't care less about the problems, if the game works for you it's such an awesome experience. I have never been shook by fiction like I was with this game, and all things considered it is my favorite eroge - matched only by C+C.

By all means, check it out whenever you're ready to take the dive.

レッツ、健全で素晴らしい世界へゴーイング！