Since I think I'm halfway (?) through, here's a longer take.
This game feels like it embodies all the (untrue) criticism that has always been thrown against the Serious Sam games and then adds a lot of dumb shit of its own.
The biggest problem and disappointment is probably the level and encounter design, or rather the complete lack of it. You know how some people say that Serious Sam has always been about nothing but arenas and waves of mobs? That has never been true because you've also had regular levels where you moved from room to room to push buttons or grab macguffins. You also had enemy placements in clearly purposeful locations. There were also 'gimmick' arenas with stuff like low gravity, bouncers, buzzsaws and the like. And of course, clever secrets that were both useful and humourous. You won't find any of this in Serious Sam 4.
The levels are literally nothing more than sets of arenas that you progress through. Even worse, the arenas are, for all intents and purposes, completely identical. There are no environmental hazards or gimmicks (really, I haven't seen even ONE), just big, empty open spaces with mobs that bumrush you. The said mobs also don't show any kind of purpose behind them - most of the times it just feels like they all spawn in one place and rush at you. You hardly even have the classic setup of reptiloids and scorpions sniping you from elevated faraway places. Instead they'll just spawn on top of the gnaars and kamikazes and whatever else and das it.
Some of the sections are also much too big for no reason whatsoever, and you'll often come across some empty open space that has NOTHING in it, maybe a single health pack if you're lucky. Normally you'd think it would be a setup for a secret, but nope. This is a gigantic failing on the part of this game - it has DOZENS of places that would have been secrets in the previous games, but now they aren't, including DOZENS of occasions where you can jump 'outside' the level and reach the boundaries of the map. But there's never anything there. At some point you just get so tired of the disappointment that you stop even trying to exploit the level geometry to look for secrets, because there's simply nothing there. And then most of the shit you find is just banal 'secret ammo cache found!' crap.
Moving on, almost every new thing this game adds is grating in some way. For example, I hate each and every single new enemy, because they are either pointless or annoying (or, most often, both). You have retarded monkey people who are fast low-hp melee enemies that appear in hordes. You have flaming dudes who are essentially no different than beheaded rocketeers. You have popamole vampires that materialise only for a moment of vulnerability then teleport somewhere else again. You have fucking sniper humans that replaced the faraway reptiloids and scorpions. And a few others that don't warrant mentioning. You could cut all of them out and the game would be improved.
Then you have the gadgets, which are essentially power-ups like serious speed from the previous games, except you carry them and they have ammo, while the actual power-ups don't exist. I personally don't like that because it's another hit against level design. Then there's the 'upgrade tree' where you can choose between stuff like akimbo or faster reloads, but again, personally I think I'd prefer these to be upgrades you'd unlock linearly through the campaign.
Then you have alt-fires on weapons. I'm torn on that because on the one hand you have an alt-fire grenade launcher for the pump shotgun, but the alt-fire for the pistol or machine gun is aiming down the sights... Also, the alt-fires are unlocked through side quests. Yes, side quests. In some levels there's an optional set of arenas you can go through to get an alt-fire module (or a new gun faster than normally, like e.g. the machine gun in the third or something level). What I hate about this is that it's very in-your-face, and that it's essentially replaced the really useful secrets with hidden weapons.
And finally, there's the deep plot and the deep lore and the deep characters. This is just so bad and so distracting, it might even be the most infuriating part of this whole game. The first two-three levels feel like fucking Half-Life 2 with npcs following you around going BETTER RELOAD DOCTOR FREEMAN. You have constant radio chatter with grating brouhaha one-liners that are never funny. You have emotional engagement where some npcs die in cutscenes and then others go boohoo. You have absolutely pointless and completely serious conversations in cutscenes where e.g. Sam has an argument with a military officer that zumg you left me for dead, zumg that is the burden of command. Oh yeah you also have audio logs spread around levels because WHY NOT. All of this not only feels utterly pointless, but again, incredibly distracting and annoying. And worst of all, it deprives this game of all the light-hearted silliness that was an important part of the previous games. Because now you have "serious" cutscenes, with "serious" conversations, between "serious" characters, and just to be clear, I mean serious as in constipated, and not serious as in 'Serious Sam'. And this shit seeps into everything else in this game - the majority of the secrets are serious, the maps are serious, the colour palette is serious, the enemies are serious.
Yeah. It's bad. At this point I'm playing this only out of a morbid curiosity how retarded it can still get, and because it's more fun to laugh at the retardation in coop than it would be to suffer it solo. Avoid like the plague.