I don't particularly think that a group of people journeying into a closed off island/civilization with strange happenings is exactly a unique idea, but Jigokuraku manages to mod such a generic synopsis into something special.

First of all, the mysticism surrounding the island gets revealed in not too short of a manner, yet not too long winded either. Plot progresses at a satisfactory pace, something that manga of this type tend to not be able to do. The introductory sequence into the island makes enough sense, as in: why do the people even journey to this death island? Overall, the beginning of this manga is very well executed, with the plot make sense. And once you get into the mysterious island, you will see some riveting imagery: truly artificial, yet terrifying. Abominations made by collecting seemingly random species of animals into a single one, and the imagery shows, and it makes you wonder: what exactly is going on in this island? This mystery is definitely explored, and keeps you wondering as to what is going on.

Of course, characters play an important role in this manga for it to be successful. If I don't really like or care about these characters on the island, or they're just plain one dimensional, I don't have this anticipation on whether or not these character will survive or not, and the manga definitely captures the characters' feelings when writing it. Motivations, life or death experiences, and bonding all change each character into different people, and the ever changing circumstances make you wonder what the characters will end up doing. Sure, there are indeed some drastic changes in character in the first chapters of the manga when they land on the island, and it might seem a little forced at times, but on the same token, having to experience something so drastic will definitely change people. I don't really think of this as a huge flaw. Of course, some characters barely change at all, but you get to know them better, which adds to the depth of the stories. What exactly are these people like? Sometimes, the answer is not obvious, and it eventually get explained as you watch them act in different scenarios.

In the case of this manga, the mysteries of the island are not exactly revealed at the end, so the plot still continues. I always find the wrapping up or ending arcs of these sorts of mysteries to be a little messy or badly executed, and this is not exactly any different. It really makes you wonder, how much plot armor characters have at some point. There are very questionable moments where they could have been slaughtered, but due to, I don't know, the laziness of the enemies, they survive and get stronger. In the end, the island is wrapped in strife and danger, and the fact that their weak selves got to the point of "getting stronger and leveling up" is hard to believe. I do admit though, this manga is not afraid of killing off some characters. It's not overabundant, as in, I still know who gets killed, and I understand them to an extent, but it's also not too little, where you are certain this character will survive to the end.

I've mainly mentioned only good points, but my biggest gripe with the manga is the "power system." It's extremely convoluted, but it takes inspiration from the trigrams, feng shui, and fundamental concepts of daoism. As a person that has read chinese cultivation novels, or "xianxia," this manga does a much worse explanation in terms of how people gain energy. In chinese novels, its real simple: take in the spiritual energy that is present in the world, and use it to enforce your meridians, flesh, and dantian. This manga takes in different inspirations from everywhere, the way dao is actually strengthened is fuzzy, and the overall concepts are just very obscure and vague, despite there being many chapters dedicated to focusing on it. How should I say this? Dao as a form of power is emphasized very much in the manga, yet it still is confusing as to how exactly its being used, and it's just the result of mixing in a bunch of Chinese ideologies that might not conform to the single concept of "dao."
Another kind of fluctuating point I want to talk about is the art. Sometimes, the art is great, like the depiction of grotesque beasts and animals, as well as the scenery are stunning, but at other times, it feels mediocre at best. Fight scenes also vary from beautifully drawn, to messily coordinated. It's honestly a mixed bag, so I can't say the art is consistently the best, but there truly are times where it shines brightly.
edit: I just finished the manga, I think the ending was quite good for this type of series. It's coherent, makes sense, and it's not a "it was just a dream" sort of conclusion. A good series overall