There are a lot of design flaws on this game, that being from a known company with experience shouldn't occur, the next ordered list is about core mechanics that are good and bad.

1. Card system
   
2. Maps and ambientation
   
3. Weapons
   
4. Complexity
   
5. Replayability
   
6. Game feedback
   
7. PVP
   
8. Settings
   
9. DLCs and custom content

1. Card system
The card system seems pretty fun in the beginning, it adds sort of a "depth" into the game, where you build your character with a desired role (more on this later). However when playing on higher difficulties, you'll encounter people with overpowered decks that only seems to get countered in "no hope" difficulty, but when playing nightmare difficulty or less, makes the experience really boring for the rest of the players, most of the time because you just make a normal deck that you expect the devs intended, have a rough time playing with your enviroment and your buffs that you remember your kit has, until you find out your holly is just straight rushing every single mutant, rider or breaker literally with a meele with a infinite temporary health healing card, that also affects you for some reason making your max health plain 60, ruining the experience for the rest of the players not even using a single gun the entire campaing.
In other hand there are fun combos like 100% accuracy (that for some reason makes a negative impact after 100% which is stupid), not that OP because you trade off ADS and resistance for more DPS or burst, I would've abandoned the game a long time ago if it wasn't for that deck possibility.
I think the goal was the game to feel as a roguelike pretty much, however you start the run with all your cards in your deck, no impact in your choices mind run, just get the cool card if you have 500 copper and thats it, most of the time the card you get in game is straight bs compared to half your build already.
Corruptions card are actually cool, but you will have no impact before starting that episode because you already made a choice in the lobby with your deck.

2. Maps and ambientation
Honestly I don't find any personality about any of the maps in the entire campaing, for me all of them where literally the same, and most of the time you are playing the same scenario (I guess is fort hope, the lore wasn't that impactful), and the only thing that change is the route after that, but I didn't feel I'm playing a different map at all, here is where I would compare with L4D2, where you could guess which campaing you're playing just by looking at the banner, maps feel different, you wouldn't mistake Dark Carnival vs Dead Center, and those are next to each other. Missions where most of the time the same stuff and boring soft locking the game on purpose just to prevent rushing (the same reason sleepers exist).
You expect a good finale and is just a mission that plays like a dancing game, not even a buildup.

3. Weapons
Weapons look good most of the time, and headshoting feels good, but there are a few problems that are the attachment and ammo system. Attachments are plain crap unless you just pick the one that buff the best stats (benefits are unclear), and most of the time it's not worth it because you now have a freaking big telescopic sight on a revolver that you cannot detach and get half your screen. And they are only worth for their stats, iron sight are cleary superior most of the time.
But the problem is worse, the ammo system straight ruins everything, you are limited with your weapon choices because you could be using the same ammunition for both your primary and secondary and that is straight suicidal, and you are guaranteed not using assault rifles if you want a clean experience with the game, because 3/4 of the players are using AR or SMG, and both ammo packs are clearly scarce, so you end playing with meele + shotgun or having a constant feeling of not having enough ammo literally half the campaing (unless you use a revolver which use sniper ammo). For me all weapons of the same class feels the same without any benefit of picking one over another.

4. Complexity
The game is bombing concepts all the playthrough, you start with cards that aren't visually different, you need to read a paragraph mid game if you wanna pick it with 500 copper, you have different stats that aren't very intuitive at first and the values are just straight random or just stack wrong. Mutants are literally the same mid game, you're just shooting in the dark you don't have a clear distinction between them unless you have one at 2m or you played enough to associate their weakspots placement with which enemy is it. Attachments, ammo system, upgrades (that no one buys), lockpicking, alarms everywhere, bird distance, when you have stumble damage, mutant cycle and lots of hidden mechanics that add complexity and just feels like a bad choice.

5. Replayability
Besides being really complex for every game core mechanic it has, the game doesn't feel replayable at all, I don't feel the impact of mutants, they just are tanky, don't have like surprise factor besides they spawn literally in the room you visited already 0.5 seconds after you leave that room. Most of the time you're playing with your decks not your weapons in game it doesn't matter at all, mutants don't have any good combos between them they just feel the same and plain, you wouldn't get a weapon choice for each mutant or something like that, the Ogre feels like a chore, literally cannot access most of the places it spawns.

6. Game feedback
The ping, enemy sounds (like when you get shoot by Hockers or Hurchins) you don't have any anticipation just damage on screen that you don't know if its coming from the riddens or those mutations until is too late. Ridden hit animations when running straight at you don't correlate with when they do damage, you have to predict when they will get to you before they are too chose (they do the swing animation with their hands but they already hit you). You don't get feedback for any of your teammates suffering besides a really low noise notification, or when mutations spawn.

7. PVP
PVP is just survival mode, not campaing PVP, you don't need an entire Act, just pick the shortest chapters of those acts, and make a PvP campaing gamemode, that would be actually fun to play instead of just camping in the same places in survival. Also you'll wait like 30 minutes before queue with anybody and probably you wouldn't play.

8. Settings
You cannot change your crosshair (or hide it in game to use a external software), you don't have fullscreen resolution by default for some reason (need to change it with a game file), the same with crosshair color, size, etc. FOV is limited, your binds are limited. Resolutions are limited in game (again, change it outside the game).

9. DLCs and custom content
Besides it being heavily inspired by L4D as TR said, you don't have any modding tool which was literally half of L4D success, DLCs are plain scam in your face, and you can play it free anyways if you queue up with a random and they have the DLCs to play already. They don't offer anything really game changing just plain retextured mutations, the only thing that should be in game and not a mandatory DLC is hives, but they dont.