To be honest, I'm quite suprised that this game has mostly positive reviews here.
Maybe that's not the worst FPS game that I've ever played (that would probably be Specnaz 2, also called in some countries Modern Warrior: Special Tactics), but it only does a few things right: there is a lot of quite varied weapons (though some of them are borderline useless - especially knife, but also the weakest shotgun ever, seriously underpowered rocket launcher or simply useless grenades), soundtrack is ok and locations' variety is also decent.

But the things this game does bad, oh boy, it does REALLY bad.
First, the game's level design is the worst I have ever seen in a FPS game. Period. No clear indication where to go, thousands of few minutes long hallways leading to a dead end, platforming sections requiring extreme precision, no map - that's just a few problems present here.
Some levels are so ridiculous, that you'll start to wonder how could living, breathing human being come up with these psychotic designs (levels "Ruins", "Shafts", "Underwater" and my "favouritie" - "Spaceport").

Secondly, the difficulty. I discovered slow motion mechanic after 20% of the game and it was pretty fair challenge up to this point (I've played on normal difficulty), so I did not really need to use it earlier. But around the levels taking place in Russia, the difficulty went to sh!t. Compulsive quick save & load after clearing each room became mandatory and even the game started to feel tiresome.
The obnoxious enemy placement making it basically impossible to clear a room without taking a damage (obviously, without cheesing) also became apparent. Also, enemies kill us after 2-3 seconds if we don't break their line of sight, if we come to close, sometimes they even kill us instantly.
After reaching Mars (last 30-40% of the game) it started to get even worse as enemies became even more tough taking basically 3/4 of the magazine to kill them and they also got rocket launchers that can easily take 90% of our health, even without direct hit.

It doesn't look like it was properly tested. I can't really recall any FPS game that I had this many problems with on normal difficulty (maybe Doom Eternal, but mostly in the beginning) and I've also finished a lot of shooters on hard difficulties (many CODs and older Doom titles).

And lastly - the story and the voice acting. I will start with the latter - it's atrocious, maybe the worst I've seen in a video game. The absolute winner is the main character - he kinda sounds like some text-to-speech app simulating bored David Duchovny.
The story is so confusing (I didn't finish the game yet) that at this point I can't really say what it's about. After every couple of missions, we get quite long cutscenes with the exposition, but they do not explain many fundamental questions about the world in the game. They are presented in a way that suggests that we should already know the factions and main characters, but they are never properly introduced. You can read the plot's description on the store page and see yourself how vague it is. It also describes Longwood as one the most important characters in the story, but as far I recall, his name was brought up once, after reaching Mars, so after finishing about 70% of the game.