Just played this in Expert with 4 friends. Playtime 2 hours-ish.
(SI: Special Infected, CI: Common Infected)

Chapter 1 - I don't like how slow the gate is opening, paired with instant respawn SI and linear infected pathing with wide open area makes the holdout a frustrating experience rather than a challenging one. You keep getting swarmed again, and again with barely any break time. Even after exhausting all the grenades you have, one mistake and you'll get overwhelmed.

Also would appreciate if you give a clear indication that the horde near the safe house is an infinite horde or not.

Chapter 2 - Same frustration as chapter 1 but with a tight sewer area this time and also the mudmen! This was the chapter where we kept on dying again and again due to being overwhelmed by the non-stop SI. For the first half of the chapter, there are too many tight corners and areas that it's hard to fight both the CI and SI, we're forced to let the guy at the front to take care of MOST the CI & SI while the others hide like a turtle behind him because we are scared of friendly firing each other. It's fun for the guy at the front, but not so much for the guys at the back.

The tank though is okay, we could see how it's well thought that you put spaces to fight it and we managed to utilize them well.

At the valve areas, again, need a CLEAR indication whether the hordes are infinite or not, it's frustrating trying to guess; will the horde end or kept on going. Not to mention the actual valves, we had to guess what they are for at first and have no idea where to go paired with a tank out of nowhere spawning during a horde, needed some trial and error on exploring what to do with practically no reward after reaching each valves, that's just a waste of time really (yes we could see the green glow for the valves, but we still had to guess how to get there).

Chapter 3 - nothing to say, same like chapter 1 for most parts.

Chapter 4 - We weren't even aware after the second half that we need to keep going to the next area after we killed the 3 tanks (or was it 2?), we were camping near the fence on the back. Wasted so many resources that caused a restart in the end. Again caused by no indication, no cue, nothing explaining what to do. You shouldn't expect us to "die first, learn the map and try again", repeating because of an "unfair" death is really frustrating.

Also the spawning of the SI for the first & second half are very predictable as they kept spawning mostly from the same spot (for first half roof, second half behind the fencing at the back). Some SI kept getting stuck on the roof behind the chimney near the ambulance if we camp on top of the ambulance during the first half of the chapter.

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You could've put WAY more temporary health items like pills/adrens and more melee weapons on earlier chapters, we explored everywhere and there are ALOT of empty spaces (and shelves lol) that would've been perfect for those. Hell or even put a T2 weapon or a hunting rifle on some spots. You made a lot of exploreable areas but it felt like a waste of time trying to explore as we aren't properly rewarded for doing so.

Overall the current version as of I'm playing is not challenging but instead really frustrating; non-stop SI, no clear indication of what to do, no cues for events, minimal relief items after/before events, no reward for exploration of the map (except the intended hidden ones), linear CI pathing with little hold out spaces for hordes.

I suggest for you to take a look at Red Tide by Monsterman94 for reference on how to handle a similar finale, they did it fantastically.

Thanks for reading, hope this would help you improve.