izigame.me

If you're looking through the not recommended section hoping to get more info about the game's flaws rather than only the things everyone would obviously like, you're in the right place
Times like these I really wish there was a "mixed" option for game reviews, as my personal opinion on Melvor Idle is very conflicted and generally tough to quantify objectively in some regards
The tl;dr is here if you'd rather just get a quick summary of my thoughts:
The game is good especially if you like runescape, but it doesn't respect your time and will likely leave you very frustrated in the lategame, or if you make "mistakes" in the early game
Nonetheless, even though I would personally say that Melvor has a lot more good to offer, than bad, and even though that would suggest I should be giving it a positive review I.. I can't really bring myself to do so?
I firmly believe that by giving it a positive review I would really undermine my frustration that came with the endgame and the final grind that really soured my experience to the point where ignoring it would show a very incomplete picture of the game
For context, I have 100% cleared the base game and gotten about 80% of the way through the Throne of The Herald DLC before quitting. I have not touched the other two DLCs.
First let's get through some of the positives:

The game has a ton of content, and the various skills you may level are at the very least different enough that it doesn't feel stale all too fast. So if you're looking for a second monitor game and enjoy "number go up", then Melvor Idle may very much interest you
The satisfaction of getting a rare drop or finishing an item you had to grind for for a long time is indeed very satisfying. Many of the hard to get items are useful so it is quite rare to be disappointed by them.
The game features a combat system which is a lot more than I can say about a lot of idle games whose whole purpose is just being a glorified excel sheet. This system gives meaning to the grind and seeing the progression in what types of enemies you face is rather satisfying
If you're a fan of runescape, this apparently scratches the exact same itch so I'd recommend it to you on principle
As far as idle games go, the pacing of the game is actually really good until you reach like.. 150 days of playtime? So considering that is more than most idle games' entire runtime it is quite nice

Now for the negatives:

The game feels very rough around the edges, despite it being developed for so long. Inside-jokes, placeholder "dev notes" and the like riddle the whole game which for someone who arrived to the party very late - is rather jarring. Not to mention the immense artstyle clash between enemies even in the same dungeons.
The pacing for 100%ing the game can slow down to a bit of an extreme degree. This is not a major problem as it is a very extra thing, but seeing how hard the expansion DLC is without the 5% damage reduction cape for full completion I struggle to see how things such as spending over a month on grinding mastery points for largely useless gear crafting fell through the cracks. This complaint will likely not land if you're a fan of Runescape as such thoughtless levelling is a big part of that game, but even for an Idle game this is a bit.. rough. It only matters once you're a solid 200+ days into the game but it's still something that really sucked away my enjoyment.
The balance of the game in terms of gear, skill and item usefulness, is very rough. About 90% of the potions you make are vastly overshadowed by only a handful of more relevant ones such as Damage Reduction or Slayer potions. Most of the craftable gear has very little purpose and outside of intentionally stunting your progression to craft every single piece, use it in combat for like 10 minutes before moving to the next, you'll likely never actually use them. Skills such as cooking have an important place in the game for about 2-3 weeks of playtime before you get vastly better sources of the only thing the skill provides. The Township skill is a relatively boring timesink, that once "complete" completely invalidates most skills as it provides you with resources obtainable through Smithing, Mining, Herblore , Woodcutting , and general gold generation as that on its own becomes irrelevant once Township is levelled up enough. I could go on for way longer about it but.. I think this is enough to show that unless you're into doing quite a bit of redundant work for the sake of completionism, its probably something that will bother you as well.
The combat, while a nice addition, is not very fun. Its about on the same level of Runescape but just boiled into a 2D set of statsheets. This is not a criticism on its own, but something to keep in mind. I didn't have much fun with it beyond just the sheer fact that it existed and it showed my progression. Combat skills are also probably the most frustrating to grind because of this.
Melvor Idle straight up doesn't respect your time. This is a weird thing to say about a genre that exists for the same of killing time yeah, however I think I can prove this point with a few examples. First of all, the standard game mode makes you lose an item anytime you die. Dying unless you're very well prepared is inevitable in the early game, or when fighting super late game bosses and dungeons. The early game doesn't matter much, even though it is rather annoying how hard it is to start your combat adventure, but the late game? Losing your Ultima Godsword or 2 Million Yak summons because of the enemy's RNG-based strongest hit + a poison tick happening on the same tick, or while doing a dungeon that is designed to not be idle-able hurts a lot. It actively wastes your time and hurts your will to redo all that progress. The hardcore mode is the same albeit more.. well, hardcore. But you know what you're in for when selecting that option at least. The second point is the dungeons that cannot be idle-able, which I already touched upon in the first point. Lastly, the Mastery system is just.. odd? The numbers feel very all over the place as some skills have a colossal mastery XP pool that you can easily and freely use to level your mastery in crafting items that are very, very annoying to farm materials for. Other skills have a pool so small that in order to not lose the 95% mastery pool bonus, you would only be able to level them to about level 80 . It's a system that I believe wasn't very well thought-through, and if it was then its designed to make you take much more time and attention in order to not hard-block your progress while idling.
Finally, I don't believe the game is very playable without the use of the wiki, guides and to a degree - mods. The game does have an integrated set of buttons that send you to the wiki, but I am personally rather frustrated that its most important guides and mechanical explanations are tossed into a webpage that contains a lot of outdated data, or asks you to mod the game to get the most out of it. The game actively throws a disclaimer and makes you check boxes that it's not responsible for the game breaking due to mods, but the guides it points you to actively ask for you to double-check their information with things such as the Combat Simulator mod. It is 100% personal preference, but it just feels rather rushed and not very well thought-through. Especially when it comes to certain drop stats, enemy stats etc., as otherwise you need to go in head-first, test things yourself , and likely rub up against the previous problem of randomly dying and hoping you won't lose your best item, as the only option for preventing that also just so happens to take away your stats.