I have played the 1.12 version the most, that said this review will also take into account newly-added improvements and 1.16 exclusive content

What is it ?

Atum II is a mod centered around an arid and dangerous war-torn desertic dimension;
It has a nice foggy desert vibe to it as if the sand itself was blurring your vision and has quite detailled nature/structures at times, making it probably one of the most detailled desert dimension mod out there. It contains some amount of lore too I guess.

What does it offer ?

Sandy lands with alternative ways for getting your hands on vanilla items and features (such as how brewing ingredients are still available within the dimension but are now different items, or how dyes can now only be obtained by using a quern , among other stuff etc.).
Challenge through swarms of enemies appearing both at night with undeads as well as during the day with warriors and bandits, with the one dungeon added by the mod which comes with mazes, traps and fights, and the need to look for food as it tends to be scarce .
It is also a quite decent-looking dimension for being desert one, as most of deserts even in plenty of video games tend to feel lacking in terms of aesthetics, while this one with its constant yellowish fog, dead trees, villages and building blocks has enough going for it to be more interesting than most.

In terms of flaws :
The enemies feel a bit generic in terms of AI, as they mostly come up to you and hit you, with little to no new mechanics with the exception of the dimension's boss giving you a status effect depending on its related deity , the assassin poisoning you on every hit or the mob infighting going in between the dead and the living. In fact I would go as far as to say the gameplay provided with the 1.12 version's boss is more interesting within the context of the structure it spawns in ( it not taking any knockback and attacking in melee forces you to sort of attack and run while avoiding the traps located in previous rooms, which is more interesting in my opinion than just having to block its projectiles with your shield ). I am not going to talk about the unused second boss as it is unused and not that interesting
The Dimension could have been more interesting in terms of looks (even though I know making a desert interesting is an hard task), and while it has several biomes they sometimes feel a bit repetitive .
I find it also a bit lacking in terms of sound design , as it mostly resorts to using vanilla sound effects, with also a lack of custom made music (regular overworld music plays inside the dimension), and there is not even something like a desert wind or ambience of some kind to follow you on your journey.

In conclusion

While not as good as other mods out there, it not having many issues in terms of performance in its 1.12 version makes it an interesting mod to be expanded on by modpacks (like adding custom structures with reccurent complex, or new mobs from other mods to spawn there using multimob or craftweaker's spawntabletweaker addon).
It ends up as a decent mod, which is good enough tutorial-wise as the original curseforge mod page gives you enough information about what the mod adds in its latest version (And the custom crafting related to the mod's new workstations are fully compatible with JEI), but some information might be missing from that. For example I don't recall the mod telling you in any way how to get the boss to spawn, you basically have to look up a tutorial for that
A shame it is now discontinued, due to this some better desert dimension mod or revamp of this one might be made one day, always be on the lookout for this kind of thing.

It is of note that dying in this dimension also respawns you inside the dimension itself in 1.12 even if you don't put a bed down, making it a good dimension to exile and banish people to (making an exit portal from within the dimension is also made harder by the fact that the surface of most biomes is too hot to keep water in its liquid state) ; take notes modded server dwellers !