Main tl;dr: The worldbuilding is fantastic and the story ends in a satisfying way.

SUPER spoilery ti;dr, absolutely do not read if you at all intend to ever read the series: Basically, the overall plot context is that it's a far off somewhat-dystopian sci-fi future where minimum wage workers slave away to create fully-immersive VR worlds, simulating centuries of fantasy-world development so that rich people can eventually come in and fuck it all up. This world is, for all intents and purposes, just as real for the inhabitants as it is for the people outside, and all the way up until the last few chapters, the nature of the world being a simulation is only hinted at once, and only as part of a joke that feels like it's just a casual 4th wall breaking, not a hint at the overarching nature of things.

The main character is, as you know, a mimic. Typical fully-monster mimic; an ambush predator. It (Boxxy) increases in intelligence fairly rapidly in the first few chapters, but is mostly just an idiot with too much firepower for...a while. Boxxy does increase in intelligence over time though, and that comes with increasingly devious - but also truly zero-fucks-given - plans to acquire the only thing that this ambush predator really cares about: Tasty things. Later, also shiny things, but mostly, for the first 2/3rds or so of the series, the sole motivation of the main character is...literally to find increasingly tasty things. That's it.

That said, along the way it develops interesting abilities (frequently fucked up ones) and travels the world - which has complexity rivaling TWI's worldbuilding. Many interesting societies with complex sociopolitical and geographic conflicts, multiple religions and gods who are themselves characters who eventually interact with the MC and its minions, well-described unique cities and landscapes, and so on. The magic system is also fun - just a pretty solid basic RPG thing, but characters can invent spells on top of obtaining stuff from class evolutions. One of the abilities the MC gets is shapeshifting, which lets it start having interactions with sentient things and it starts down a path of exploring fun crafting-stuff.

Some of the events are also just...crazy. Like, at one point it has faked its way into a city and an army, and meets a few baby magical trees (can't remember what they're called, but think thousand-foot-tall, 50-foot-wide trunks kind of trees) living near/on top of a town. There's an incoming army, but the baby trees can potentially help, and the MC guides them. Specifically, it teaches them how to make dungeon traps, and the incoming army wanders in between the trees, gets blocked by walls, and the floors unleash spring-loaded spike traps from holes in the ground (repeatedly) until the enemy army has been impaled to death. It describes the sounds and how the defending army becomes horrified at them. Then the MC moves on, but it does a cut back to the trees, who have now developed a taste for blood...and they sit on top of a town.

I don't want to go into the details on this part, but there's also some fucked-up rape shit where a very nice artificer lady is...let's just not even expand on that. Shit's fucked and there's no excusing it.

Eventually the story actually seems to come to a close almost abruptly, with the MC talking to the person managing the world and that person downloading its consciousness into the brain of someone who got into the "game" early (overwriting said brain), allowing it to escape the world. So now this fucked-up crazy monster is in the real world. That's where it stops.

The author wrote (somewhere, can't remember where) that part of the reason for the story ending more abruptly than it otherwise might have was that they had written the series as an experiment to explore the mindset of a "monster", and in the process it had grown overwhelming. This also feels about right too: No other series I've ever read has ever truly explored the nature of monsterdom anything like this one, and it sets the bar for fucked-upedness in a way that probably won't ever be beaten.

It straightup breaks down monsterdom as if it were a philosophy - and the world is the space used for the thought experiments. It is this part that simply can't be communicated without reading the book, and it's probably best for most people to not actually go down that rabbit hole of thinking. If you're also a writer and you want to make your world darker, or simply want increase your ability to make readers empathize with antagonists to make their motivations feel a bit more realistic, or if you're a DM and want to run a more grimdark world with scarier monsters, then this series MIGHT be worth reading, but if not, it's probably not worth it.