Alro

This mod has Microtransactions!

Even if that wasn't explicitly against Minecraft's terms of service - (Which it is) - I still hate the idea. The fact that this mod has been able to get as popular as it has without triggering any form of retribution will encourage others to follow in its footsteps.
A huge part of what makes the Modded Minecraft community so special is the capacity for anyone to assemble a modpack just how they imagine, these massive experiences of mods from hundreds of creators unified under the creative vision of a single individual, or small group, and then becoming accessible to everyone. A paywall of any sort blocks that opportunity on both sides, creatives cannot create, players cannot play, and even if the paid elements here are just cosmetics, a mod with over 100 million downloads breaking ToS will invite more.

The mod works well or whatever, but you can use E4MC for a similar experience, without all the cringe, or set up Zerotier One, which is a little more difficult, but will also work for other games.

It appears that the next vanilla update will be adding a similar system, and I hope it kills this mod. Do not give them your money. Donate it to any other modder, or to charity, or buy Sea of Stars, that game's great.

You have no idea how many modpacks I've played where progression gets completely snapped in half because JER gave inaccurate information to the pack developer about loot in chests or mob drops. No other JEI addons have this problem, and while yes, there is a command to update the information, in practice, it crashed the game and does nothing. My understanding is that even if it works, it only does ore generation graphs.
The mob drops UI looks pretty good, though it should be accessible from the mob's spawn egg.

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.

Gives you all the information you need, while looking like a part of Minecraft's UI, lets you customize practically everything, there are seriously 7 pages of options, you can freely click and drag the map into any part of your screen, and can show icons for all mobs, even allowing mods and resource packs to add custom icons. The Craftable Waypoints addon doesn't seem to be maintained anymore, but it's a very cool addition. Even features I'd think to request, like automatically waypointing your portal when you enter a new dimension, could well be options I simply haven't found yet. Updates come extraordinarily regularly, even on game versions that are not popular. Dropped items are rarely shown by other map mods, but they're incredibly useful in all kinds of scenarios, and actually allow you to play the game without the ugly hitbox mode enabled sometimes.