Note that this is just a short, cut version on YouTube of the BTS/making-of videos, the full 20-ish minute versions of which can be found as "Special Clip" on Khemjira's iQIYI series page.
How 'horror' is this? Cuz im a huuuuuge scaredy cat
Honestly most of the more traditionally scary-type stuff is packed into the first episode; if you can manage to get past that ep. 😅, it actually does get way less scary. (I mean, don't get me wrong, there are obviously still scary/life-threatening/etc. bits in all the other ep.'s afterwards, lol, but it's just that from ep. 2 onwards it's definitely not constant; a lot of the runtime of the show actually starts to just be more like character work and intriguing backstory/history stuff.) And as for the ghosts themselves, most of them look obviously not-alive, but it's not gross (well, again, aside from ep. 1, lol; they are a little bit more gross there 😅), it's more just like people who are really pale with visibly dark veins lol. Obviously, you are the only one who can decide how much you can handle 😅, but again, the very first ep. is the scariest one by far, so if you can make it through that, you're totally good for ep.'s 2-4.
LOL at some of the comments here saying that the romance 'came out of nowhere'... seriously? I mean, I really don't like love triangles 😅, so I'm not defending that part of it (though it really kinda ended up more just like a mixed-gender harem around the ML than any sort of triangle anyway, LOL...), but if you've read like literally any shoujo manga ever, you would not only have already assumed that this show would have a romance between the FL and ML right from that duet scene that opens episode 1, but you ALSO would have clocked Sakamoto's tsundere backside IMMEDIATELY (I mean, come on, it was clear that he was a tsundere by episode 2 already, and his crush on the FL was glaringly obvious by episode 3, LOL)...
Let's just get to it: THE TIME TRAVEL. Are you kidding me?! So, we go through all that emotional turmoil, all…
(Pretty sure the future meet-cute after the credits was actually with the twin, not Victor - the cheek mole can distinguish.)
That said, yes I think we're pretty much all agreed that this was really bad, and honestly the uncut version is even worse... in the full uncut version, the main character himself actually dies as well, for no good reason whatsoever, AFTER the reset. So the series literally ends with the main character dead.
Please note that this is the theatrical cut version; however, there is also an uncut version that was released on home media that has 30 more minutes of footage (and makes much more sense as a result)! (Just submitted that entry to the database~ :) )
K-Drama 
And then they released an 8th episode a few weeks ago: https://www.youtube.com/watch?v=LnJPUDWYFXc
Apple: https://apps.apple.com/us/app/shortime-stream-k-drama-show/id6502581355
That said, yes I think we're pretty much all agreed that this was really bad, and honestly the uncut version is even worse... in the full uncut version, the main character himself actually dies as well, for no good reason whatsoever, AFTER the reset. So the series literally ends with the main character dead.