I think there's something of a divide between people who use this mod on newer versions and people who prefer the older ones. While I'm not here to start a version war, I do believe Quark is an example of a mod that has only gotten better as time has gone on.
The original versions of Quark had a simple goal of introducing "Vanilla+" content. This was before the explosion of popular and extremely well-made "Vanilla+" mods, and Quark was something of a pioneer in that respect. Newer versions of the mod show this age in how they, compared to older versions, prioritize features and thoughtful integration into the vanilla ecosystem. A post I often come back to is Vazkii's old release notes for the 1.18 version, where he speaks about some of the design decisions in slashing several features from the mod during the updating process.

For quite a while now, one of the most common bits of criticism leveled at Quark was the sheer amount of extraneous and extremely niche content included in it.
While "if you don't like it, don't use it" is a common answer to these bits of critique (especially given Quark's configurability), I think that dismissing them like that is reductive to the problem at hand - the fact that the mod does, in fact, contain a lot of features that would get you laughed out of a Mojang meeting if you tried to suggest them.

Looking through the old 1.12 version set, I'm struck by two observations: the number of features which just exist in Vanilla now to some extent or another, and the degree to which the "extremely niche content" shows up alongside the mod's most useful additions. For example: I literally cannot live without the Reacharound Placing. I forgot hotkeying inventory to offhand was once a modded feature because I use it so much. Meanwhile, the Lit Redstone Lamp is an extremely boring item. (Sorry to any Lit Redstone Lamp stans out there.) Older versions of Quark showcase this type of imbalance more severely, where a lineup of forgettable building blocks is included alongside game-changing quality of life features.
I find that I'm consistently very happy with the direction this mod's development is going in. It adds new features more carefully these days, and older ones are updated to work more seamlessly with new vanilla features. This is why I can strongly recommend the modern versions of Quark, especially those after the aforementioned 1.18 release. The older versions feel more like a relic of a past era, but they were made for a game with less overall polish and a fairly different design system.
All this being said, I have never understood the criticisms of "bloat" when you can configure every feature in the mod. The configuration menu means you can easily click through and remove anything you don't like if you already understand the mod, which you have to in order to know what you want to get rid of. This combined with the trimming of some lesser-quality features makes this mod a perfect addition to pretty much any pack, in my opinion.

I remember seeing a meme a while ago, "What mod adds ____? Its Quark its ******* Quark" I couldnt find it but that should show how large of a mod quark is.

Quark is the epitome of a bloated "QoL" mod that people who dont know better install, then have to spend 5-10 hours troubleshooting, debugging, and configuring the completely random additions of quark, with the additions themselves being very poorly communicated in game.

Quark also doesn't describe the features it adds on its mod page, again leading to people jumping to reddit to ask questions about the mod of mods.

I also also talk about Zeta, the library for Quark 1.21.1 and 1.20.1 only. Zeta has 12.3 Million downloads, despite being pretty new. Zeta adds nothing to Quark or otherwise. Its many undocumented files and mearly wrappers for the existing vanilla files. Ex: ZetaFenceBlock extends FenceBlock, and ZetaFenceBlock is 5 lines of new content. Those downloads give them a nice chunk of change for basically nothing. But dont worry! They also proudly advertise their patreon and ******* tshirts on every mod page.

I suggest installing Quark as way to discover a specific feature, then to uninstall the bloated trash and find a single mod that does that feature you want.

There are features in Quark that are downright difficult to play Minecraft without, and really ought to be added into vanilla as soon as possible.
Linking items in chat is practically required for playing multiplayer, the option to build on the far side of blocks by sneaking is a life saver, Glass Item Frames are the decoration block of the century, not to mention, all item frames work on the top and bottom of blocks, the tooltips for food and armor have information that is extremely useful and cleanly presented, and I personally end up making a lot of use of the emotes and fairy circles.
However, not every feature is on this level. The biggest standout are the ghosts, which are three white squares that make random sounds, rather than having any of their own, and just kind of spawn around in the nether. The autojump keybind is exceedingly annoying, and I'm very doubtful that anyone in history has pressed it on purpose. The animal texture variants are hideous, Quark somehow managed to make a frog that I don't like, and that takes effort, the Nether is full of normal melee monsters, which isn't what the Nether is designed for, guardians free-spawning at random in the ocean is an unhinged idea, axes fast-breaking leaves instead of hoes leads to you accidentally wasting tons of axe durability trying to mine wood, and the vast majority of content comes in the form of mid-tier Building blocks that you won't use, because you're playing modded, and there is no chance your other mods don't have better options.
For a mod based around customizability, Quark is remarkably bad at having the kinds of options I'd want. For instance, I don't like the normal animal variants, but I love the idea of rare shiny variants of mobs, but they cannot be enabled separately, you get one, you get both. The Tetris enchanting system is really cool, but other mods rely on enchanting to work the way it does by default, I'd love to just have a variant of the enchanting table with a different crafting recipe that does the tetris enchanting, but instead you cannot enable it without removing default enchanting.

Overall, as stated in the title, one of the most essential mods out there that I never want to play without, but configuration feels less like tweaking it to my preference than hitting it with a hammer until it behaves.

Very good demonstration on configuration
Has alot of valuable features

As a modpack dev, having mods be as configurable as possible is a massive help, if not a necessity for many mods. Quark really goes a step beyond and adds pretty much every single configuration possible, even adding a custom in-game menu for it.

The features Quark adds are comparable or just straight up better than other mods' implementations. The backpack is a favorite of mine. It's such a clean way to extend the player inventory, not just add a completely separate inventory (good for immersive packs, maybe not for tech or kitchen sink packs). Also love the totem of holding in place of other grave mods. It's simple, intuitive, and configurable. Quark really helps keep the mod list down and all it's features feel very balanced.

If you're on 1.12.2, highly recommend Quark RotN Edition for even more config features.

Despite this mod being a ubiquitous part of Minecraft for a while now, I never really realized just how many features this mod adds, it is surprisingly vast, while there are only a few that stand out to me and fewer that are ground breaking, the sheer amount of quality content is awe inspiring. I'll also say if the client-side features were spun off into it's own mod, it'd be a daily use mod for me.
I honestly believe Minecraft as a whole owes a great debt to the Quark mod. It set an unrealistic standard for many of the features Minecraft would end up adopting, such as stalagmites and stalagmites which look so much better in quark, a believe I hold so strongly I went and made a resource pack about it. or ofc the famous frog who is the best frog in all of Minecraft and I'll kill for him.

The config for quark (editable ingame, from the main menu) may be one of its best features, containing the ability to disable entire modules alongside tweaking specific additions. Their website clearly explains what each addition does, for every mc version of quark, in case you’re ever confused. In my opinion, these features alone make this mod stand leagues above others for casual players, modpack creators, and anyone who enjoys small, fun, (sometimes very QOL) features that mostly feel like they’d fit into the base game. It gives you the ability to completely customize the mod and cultivate it to your liking or needs. It’s also hard to fault this mod for that same reason: it’s trivial to remove features you don’t enjoy.

It has a lot of QOL features, but one thing I don't like is the bloat. It adds a ton of useless stone types that clog the inventory.