In short:

It's unforgivably bad. Not playing it is more fun than playing it. It's not enjoyable having your options for potential builds cut down to a third just for a specific mission and the difficulty feels artificial. People quit out upon joining it to begin with. At least cutting it from the game would save them the extra steps involved in avoiding it entirely.

In long:

Pre-patch, the phage tree was infamous for being needlessly difficult. There's not many missions where a certain build is 100% necessary. Some mutators like high-intensity, perhaps, but you could still caveman brain through those. But then comes the phage tree. It was too open, enemies come at you from too many directions, and there was never any breaks from the unending onslaught to actually do the objective when it broke down.

If you didn't have a player who had some kind of cloak, shield, or defensive ability, it was pretty much unwinnable. You're not going to coordinate with pubs. Even with a coordinated team, somebody needs to notice and remember the phage tree is the most pointlessly difficult mission in the game. And because of the incomprehensible decision to not let you change your ability loadout if you join a game after it started, you can't even adjust your build upon seeing the mission if you use quickplay. Players would frequently abandon it entirely if they got dropped into it, knowing what was to come.

This is on top of the mission being extremely lengthy and with a scanning event just before it, arguably one of the hardest scanning events in of itself due to enemies once again attacking you from every direction on a map built like a maze. The phage tree would still be bad but at least tolerable if it came at the very beginning of the stage. Instead, the hardest part is saved for last, where you will already be worn out, missing some resources even with the supply dump before it, and too far in for most players not to stubbornly hold onto a slow defeat.

Post-patch, they added some fences and junk to...act as cover? Except it had the opposite effect. Gunner enemies now sit behind fences or high ground and plink away at players and the objective will occasionally (but randomly) force you to stand out in the open while you sit around hoping either your teammates cover you and there's no gunners. So they effectively added cover for the enemies, not you. I can only assume there is a forum where NPCs can complain about game balance, too.

The same problems from before persist, as well. There's no breaks, the objective forces you to go out into the open, and enemies come from literally everywhere, including specialists and elites like maulers who can one-shot anyone doing objective, or muties who can grab anyone doing objective, and snipers who can obliterate anyone doing objective.

The most baffling, mind-bending thing was the decision to include a brand new, entirely unique minigame to the objective in the worst possible mission. People don't play phage tree, and the objective is at the very end, so even less players will see it. So now you've given players a minigame that isn't intuitive that they need to figure out for the first time, all in a situation which already creates the hardest environment to do any minigame in, familiar or otherwise.

Since the minigame itself is exclusive to this stage, I also consider an extension of the phage tree, so I'm criticizing that, too. In it, you get some circles and have to click on the green one. This is already stupid and entirely luck based, even more so than the default minigame, but it's also slow and clunky. You can't quickly poke at a dot and immediately swap if you see red, it holds on what you selected just to say you're wrong and makes the whole experience even more frustrating. Worst of all, it doesn't save any progress, so in the extremely common event you get spanked by some random mob as they walk past you, you get to start all over.

I have seen games go on for around 20 minutes just for this specific part, teammates slowly being whittled down and refusing to give up because of how much time they've invested in this stage, on top of the roughly 20 minutes it already took to get there. There is absolutely zero compromise for the player whatsoever. No advantage is given, be it cover, high ground, chokepoints, or anything else seen in other finales. You can't even go back to heal because the mission starts with a dropdown.

I'm not going to put on my "game designer" hat and pretend I have any expertise in balancing. All I can say is, as a player, I think this finale would be reasonable if enemies all came from a choke point so you realistically protect the player doing the objective. Or if there were breaks in between hordes so you can do objective. Or if the objective was done in chunks so you can do the objective. Instead, you can only do the things which brute force every objective to an excessive degree, except it's entirely necessary here. This stage would be difficult all on its own if it was just a hold out, but you instead tag on an extra requirement which makes it borderline impossible without specific builds.

As of right now, with few devs to actually invest any time towards fixing this thanks to Fatshark's summer break, I'd honestly prefer if this mission was axed entirely so it doesn't show up. Players already ditch the mission on sight if they get put in it by matchmaking, so cutting it entirely would be an improvement since it doesn't pollute the pool of potential stages.

Until then, leaving this level completely unchanged, all it does is act as a means of introducing an overwhelmingly sour taste to players who naively believe this war crime of a stage couldn't possibly exist in a modern game. Cutting it until something is added where players can at least squint their eyes and pretend there's something fair about it would be the most realistic choice, in my opinion.