I agree that on the normal difficulty = easy, the game can be finished in about 5-6 hours. But once you ramp up the difficulty to hard or inferno, you suddenly realize that you definitely have to start ranking up your player if you want to have any chance of getting through it, and of course, that's going to take a lot more time to do so.
I was the one that originally posted what Gametrailers had to say about the game, and after playing it now for approximately 25 hours, I totally disagree with almost everything they wrote. I do agree that it doesn't have the amount of enormous monsters that 2017 had, or the more open locations, such as the beach or mountains, but it does have the 3 basic things that makes it what it is. It's simple, it's fun, and it's totally addictive, just like an old-school shooter. I know I can't stop playing it, and the Campaign remix mode is a great addition, that changes the enemies you face during each mission. It's an easy 9/10 for me, and I definitely see myself trying to finish the game on inferno with all 4 armor classes, and I'm sure that will take me right through till September to do so. LOL