izigame.me

I really liked this game, but I can see why it wouldn't be everyone's cup of tea.
First and foremost - it's far more enjoyable and replayable with a group of friends who are looking to beat the game on higher difficulties. If you're just playing solo or aren't interested in beating the game on nightmare or no hope then I could see it getting old fast.
This isn't Left 4 Dead, sadly. But it does build on the ideas of L4D - a coop shooter with replayability.
What it does well is the replayability side of things, creating niches for different weapon types, and the overall polish.
The card system and the rogue-like elements (randomized item and weapon drops for each run) lend themselves pretty well to the replayability - I've lost count of the number of times I've replayed Acts 1-3, but a lot of them feel distinct because of the card builds I've experimented with, and because of the different weapons and attachments.
The different weapon types having different niches really works well. Back when L4D2 released the meta in campaign versus was to run AK47's, and using the auto sniper was suboptimal - less dps, difficulty with horde clearing. There wasn't much reason to use pistols.
On the higher difficulties in B4B, each gun has its place: snipers for long range mutation and boss dps, shotguns for short range dps (and horde clear in a pinch), assault rifles and LMGs for horde clear at any range but inefficient for boss/tallboy dps (ammo becomes an issue), and SMGs for horde clear with the lowest dps player on the team which is usually the medic. And usually everyone uses pistols outside of hordes to clear zombies - otherwise you'd run out of ammo real quick, even if people are sharing their ammo.
Melee is a great style as well, though it isn't as viable in no hope. It is overpowered on the lower difficulties since it has so much sustain. But I like that even as powerful as melee is for hordes and lone mutations, it can't quite handle everything by itself - ogres, spitters/hockers at range, retchers etc.
Each weapon ends up having a fairly distinct playstyle as a result, and in a team it lets everyone fill a role for targets.
The cards add a huge amount of replayability - just a few cards can really change the way you play. Pure DPS with reload speed, damage with copper, damage with speed, melee tank, melee speed, hybrid melee healer (I have a funny memory of me being a melee Holly outhealing our doc with bandages, and healing heaps of trauma with white items), no aim down sights builds if you want the counterstrike experience - the list goes on.

Now for the downsides:
Unfortunately you need to grind a fair bit to get all the cards, and I think a lot of players stop playing before they unlock all the cards. So they don't get to experience the full range of builds with cards.
Since release, the devs have allowed people completely new to the game to queue for nightmare in Act 1, which is just bizarre - it leaves a lot of new players with a bad impression of the game. Even after they made nightmare easier with patches, it is still very difficult to beat act 1 on nightmare for people completely new to the game because the beginner deck is trash. The advice I've seen on discord and reddit is to farm points in easy and veteran until you have enough cards to build a proper deck before playing nightmare, which I strongly agree with - it's not a matter of skill or experience (though it does help immensely), but rather having the right cards.
A few of the guns just aren't balanced very well. The M4 for example has awesome design - it sounds great, the model and animations are great - but the damage is WORSE than almost all the SMGs (why does an UMP do 40% more damage than an M4?). Once you dive into the numbers you'll realise that in most weapon categories there's a best-in-slot weapon type for the higher difficulties, or at least worst-in-slot. (If I had to name the worst-in-slot weapon for each category, it'd go: beretta M9 (burst strangely does way more damage and therefore is fine); MP5, M4, super90, M1 (yes I've heard the m1 actually has more dps than the phoenix and barrett at blue/purple, but ammo is a big concern early on in high difficulties)). Fortunately most of the other weapons are fine balance-wise.
The voice acting and the lines are cringe. The way they tried to add depth to the characters - random moments of them opening up, sometimes just before a horde - just feels forced and awkward. I think the devs tried to make the game a bit more serious than L4D since the voice lines are all serious and lack humour, but I believe this made the B4B characters less likeable: all of the base characters tend to be snappy and negative towards each other, and a lot of the notable NPCs such as general Phillips and doctor Rogers aren't very likeable. Whereas in L4D1, they largely avoided the raw emotional dialogs and went with randomized dialogs that were based on the map location, which while serious still had a certain lightness here and there ("I hate hospitals... and doctors and lawyers..." "Francis is there anything you don't hate?" "You know what I don't hate? I don't hate vests."). Most of all, the friendly dialog they had with each other helped people connect more to the characters - I reckon my friends and I are more attached to the L4D characters than to the B4B ones.
In terms of PvP there's no campaign versus. It's a big negative for me, since that was such a unique PvP mode and it was part of what kept me coming back to L4D. Swarm mode is basically L4D survival versus, which while well-polished isn't my thing. While the points system is well done and adds a bit of depth and strategy to team allocations of points, it's conversely quite frustrating to play with newbies who don't put points into anything at all while playing against a coordinated 4 man stack.
Lastly, the devs have stopped development on B4B to work on their next project. It was nice that they released so many patches up until then, and after their initial few patches of nerfing everything without buffing anything which made the higher difficulties just feel oppressive, they started making some good changes that felt good to play with. So while I'm a bit disappointed that B4B is essentially at the end of its cycle, I'm pretty happy with the game and the patches that the devs have given us - though it's got a few rough edges, it's got great depth and quite enjoyable with friends.