Wayfinder is a great Co-Op (and Solo!) action RPG. It may have had some troubled history in the past (which is well reflected in Steam's overall 'Mixed' reviews), but don't get discouraged - as of today, the game is a complete experience that's well worth the price and is a lot of fun.

Spoiler-free review after 100% Achievements, All Nightmare Mythic Hunts (Endgame content) and Nightmare Challenges.
<=Positives=>
Combat system.
Combat system, albeit quite simplistic - you have a jump, dodge roll, light attack, strong attack, 4 character-specific abilities and a weapon-specific ability - still manages to weave in some complexity with animation cancelling and very specific build-enableling combos.
For example, Wingrave, being of one of the 3 characters you can start with, has a passive ability that heals him and nearby allies whenever he performs a combo finisher move, and weapon type "Sword and Shield" allows you to charge heavy attack during light attack combo to perform shield slam, which counts as a combo finisher. This 'tech' allows you, whenever you are in need of healing, to skip going through an entire combo sequence and instead just light attack - charged heavy attack once to reap the benefits.
Each weapon type in Wayfinder has some kind of 'tech' you can discover and (ab)use in your build, which feels like an extra dimension in your character build progression: you are not only increasing your stats, but also improve as a player, discovering and coming up with interesting strategies.

Character Building.
Build variety in Wayfinder is quite high, despite the game being (relatively) small: you have 8 characters, divided into 3 distinct classes: Warmaster (Warrior), Survivalist (Rogue) and Arcanist (Mage). Each class has their own version of a passive tree you can spec into, with special, class-specific, bonuses to certain weapon types. On top of that, each *character* also has their own passive skills you can invest into. But that's not all.
There are 4 weapon categories: Guardian (Sword and Shield), Executioner (Two-handed Sword, Mace and Scythe), Twin Striker (Paired Daggers) and Marksman (Single shot Rifle, Burst Rifle, Shotgun and Railgun). Inside those categories every weapon has its own set of stats and affinities, and a custom, basetype-specific, weapon ability. But that's not all.
Perhaps the most impressive aspect of Wayfinder's itemisation are so-called 'Echoes' - they are basically a light version of soul cores from Nioh 2 - each enemy type has their own echo they can drop, and bosses have their special echoes. While regular enemy echoes give you stats, boss echoes straight up give you a part of that particular bosses moveset, giving you even more options for build customisation.
There are other aspects of itemisation, such as character-specific armour and jewellery - I'm only going to mention them, but they do add yet another layer in building your character.

Endgame Content Variety.
There are 3 main activities in Wayfinder endgame: Dungeons, Bosses and Overworlds. In this review, I'm mostly going to be talking about Dungeons.
Overworlds are mostly static big maps you traverse during the campaign, featuring their own set of events and bosses - with some, namely "World Bosses" being quite tough and rewarding.
Dungeons consist of procedurally-generated areas based on the tileset you pick, and come with specific events exclusive to the tileset. Variety is a big one here, because even after 90 hours in-game, I still managed to stumble across things I haven't seen in the dungeons before. It does help that you can customise your Dungeons further by adding so-called 'imbuements' to them.
Imbuements are special items that act like area modifiers to the Dungeon they are applied to - without spoiling much, they can be used together in order to create special combos that completely change how an individual modifier behaves. For example, if you were to add Greed Imbuement (adds piles of gold to the dungeon, but makes you take extra damage for picking them up) together with Shadow Imbuement (makes monsters enrage at low life and explode after death), you get special "Golden Cache' modifier, spawning sealed treasures, that can be unlocked by being hit by a monster on-death explosion . These combos add a lot of variety to your dungeoning, as well as reward you with special items for completing runs with them active.
Upgrading your character in Wayfinder, like in many aRPGs, comes in a few forms, so, during the endgame, you would be farming different dungeons and bosses in order to gather upgrade materials, get special weapon and jewellery drops as well as echoes.

These are my main positives of Wayfinder - I could also talk about pretty visuals, overworld and dungeon design, as well as enemy variety, but those topics can be pretty spoiler-heavy, so I'll only mention them. Moving on to negatives.

<=Negatives=>
Subpar Performance and Stutter.
Unreal Engine #StutterStruggle.
There is a lot of stuttering during gameplay and it's pretty annoying, especially early-on, when every visual effect briefly freezes the game. It does get better after awhile, however - there is also fairly noticeable traversal stutter in Overworlds, and that one doesn't go away.
It's also worth mentioning that I've run into quite a few crashes and some cases of weird CPU utilisation - the game would randomly jump CPU usage from 40-ish % to 90-100%, greatly tanking the framerate until restart.
Overall, however, I can't say performance for me has been too terrible: with mostly default (medium) settings, resolution scaling 100%, DLSS Quality, 1440p, my 12400f - RTX3070 combo's been giving me 100-140 fps in dungeons, which isn't too bad. Overworlds are a different story, with some places in Frostmarch giving me 50-60 fps.

Weak Side Quest Design.
Most side content in Wayfinder can be summarised as 'Early 2000 MMO Quest' - fetch quests galore! Doesn't help that pretty much none of the side quests are voiced in any way, so even *if* you, theoretically, wanted to engage into the story of them, you are forced to read volumes of text after text in neverending dialogues between quest objectives mostly amounting to killing a pack of monsters, travelling back to town in order to talk to an NPC, walking to talk to another NPC, loading into a different zone to, once again, talk to an NPC and finally get the quest reward - XP and generic mats.
Thankfully, this content is 100% optional, but still worth mentioning.

Grind For the Sake of Grind.
"What, an aRPG is grindy? No way." Yeah, I know.
The amount of materials required to upgrade characters and weapons feels disproportional to the game's content scale.
For example, in order to max out 1 weapon base, you are supposed to get 18 of crafting resource, exclusive to a particular boss - you get 1-3 of that resource per kill (1 being very common, 3 being quite rare), which means at least 10 kills of the exact same boss - and that's only one of the components required. Why isn't it at least a couple different boss resources? And it isn't a one-time thing, if you decide to use a different weapon, you'd need to farm materials again, sometimes even from the exact same boss you farmed 10x before.
The real elephant in the room, however, are the boss-specific jewellery droprates - they don't feel tuned for singleplayer experience at all. Each boss can drop several pieces, from different sets, and while they are powerful and definitely not necessary for the game's hardest content, they are really rare, making you kill boss 50 times or more.

These are my main negatives of Wayfinder - I could also talk about dragged out story, bugs, not-weak-but-rather-generic character design and encounter balance (burning ground bad), but those topics aren't as interesting or important as the ones in the review. And frankly, this review is long enough already.

A hidden gem. Game had a super rough start but changed from a cashgrab mmo mtx nightmare that failed to launch, into a legitimately fun labor of love. Ignore the mixed reviews from the past, they don't represent the relaunch of this game after they split from their greedy publisher. Now you pay once and get all the content, all the fun and all the game has to offer. It's well worth the price.

Game's great but the traps and environmental hazards need a hard nerf because on the higher difficulties they instantly kill you which is extremely annoying because they're everywhere, can be well hidden, and can be hard to avoid. Everything else for the most part is fine, it's just that particular issue that gets in the way immensely.