Ignoring how stupidly repetitive it is to go from the same hives over and over and waiting around for the drill to finish drilling, about 25% of the way through the map, you'll get hit with this really stupid difficulty spike where the game just throws assloads of Cryptids at you with extremely little breathing room. It's like if you played Zombies but by round five all the zombies were as durable and numerous as round 15 zombies. Half the Cryptids are just damage sponges that deal stupid amounts of damage, including one that teleports around rapidly that has as about 75% of the health as the Rhino, the resident L4D-Tank of the mode.
They clearly never balanced this shit for solo play because I can make it around 70% of the map before I get overwhelmed by the sheer fuckton of enemies they throw at you. You can upgrade your gear mid game, but even when I max out everything, I still get swarmed and die over and over again. I know I'm not the best player in the world, but holy shit this mode is fucked when it comes to balancing, at least when you aren't with other players.
The game has upgrades you can buy from the menu, but they're all worthless. Shit like "do 5% more damage" or "put an ACOG sight on a revolver". The only good shit you can buy requires hundreds of of the mode's premium currency, "Teeth", to acquire as if this was some P2W shit, except without the ability to actually buy Teeth with real money. The only ways to get Teeth is to complete a map, kill 300 Cryptids or complete a map with a Relic.
What is a relic? Debuffs. That's all they do. They make you take more damage, they give you less money you can spend, etc. They provide no advantages or Buffs to balance anything out. The only thing they do is give you more Teeth when you finish a map, but the difficulty on normal is so stupidly ridiculous that good luck completing the fucking map at all, never mind with this shit on. And Relics are the "only" things you get for leveling up. See, Extinction allows you to level up like in multiplayer, but the "only" things you get for reaching max rank is a Relic.
The story of Extinction also sucks hard. It tries so hard to be serious and grounded with this overcomplicated narrative of "this girl might be valuable, oh now we have to kill her, actually keep her safe, actually she's bad" and it's so trash. The "main villain" of the first half of the maps is some British SAS guy who smokes, so coooool... It ends with all of humanity being wiped out by the Cryptids and the girl escaping with the last of humanity on a space shuttle, but surprise, the girl is actually evil and is totally going to either kill or enslave everyone left on the shuttle.
Seriously, Extinction fucking sucks. All those people who say Extinction was the only good part of Ghosts can smash their head into a doorframe.