Well, just beat the campaign. As someone who sucks at FPS games, was loving it until the dual hell guard boss fight which was annoying. Also the build up and the fight challenge made it seem like that was the final stage for the crucible and I was expecting a final boss...but then there was more. The sections after the dual hell guards (Vega/Cooling Stations/Argent) were a bit going through motions because at that point you'd done all this already, pretty much fought all these enemies in big group challenge battles. The 2nd hell section (or was it third?) that lead up to the hell guard bosses was really long and tons of intense fights, so the stuff after lacked the intensity a bit. Was actually surprised the final enemy fight room was pretty standard enemies and IIRC had a quad damage powerup.
Final boss was fucking hard. Almost quit and lowered the difficulty level after like 10 tries/30 minutes. Stuck with it and barely beat him with like 5 health left.
The game is engineered to let you take advantage of the pick-up availability and customization options to mitigate the overwhelming odds in a means that makes sense to your play-style.- There is always at least one power-up available in every combat arena in the last half of the game.
- There is always BFG ammo available to encourage you to use it and not save it.
- If you're low on ammo, the Chainsaw is your instant ammo replenisher for all your guns and it's an instant win against any enemy.
- Remember you have grenades that automatically recharge. The Siphon is your go-to on harder difficulties.
The bosses also have weak points that encourage you to play into that. For example the Hell Guards have attacks that can be interrupted so a faster firing weapon will be most effective with the Plasma Rifle absolutely dominating them. The Spider Mastermind (final boss) also has BFG ammo in the arena right before and in the boss arena itself so if you play it smart you have at least 4 shots to use against it which dramatically cuts the health down.
As for customization, the game rewards you if you seek out Runes, Elite guards, and mastering your weapons.- Prioritize maximizing the Power-ups with your Praetor tokens, as the last half of the game is pretty liberal with power-ups and at max upgrades you have double the power-up length and it sends a devastating shockwave out when the power-up runs out. This is insanely overpowered with Berserk and Quad Damage.
- Prioritize boosting Ammo capacity with the Argent Cells. Having max ammo lets you lay waste to everyone and increases capacity of the BFG and the Chainsaw. More importantly a larger ammo pool with the Chainsaw lets you kill bigger enemies which means a literal instant-win button against a Hell Knight and Mancubus.
- Find the Rich get Richer rune in Lazarus and equip it. Once mastered you have unlimited ammo if you can maintain 75 armor or higher.
- An awesome Rune loadout is combining Rich get Richer with the Armored Offensive (found in Argent Facility Destroyed) and Dazed and Confused (Argent Facility) runes, which is awesomely OP since enemies remain in stagger for much longer and every Glory Kill gives you armor.
- Prioritize mastering a few weapons such as the HAR micro missiles, Rocket Launcher remote detonate, and Gauss Cannon siege mode. Combining a mastered HAR micro missiles with a mastered Rich get Richer rune gives you unlimited ammo with no missile re-rack so you can literally walk around the rest of the game without ever letting go of the fire button. That results in a non-stop barrage of missiles flying out of you which is fucking hilarious.
tldr; great game, best fps combat and movement in ages, incredible animation work, overstays its welcome just a tad imo.
People whine endlessly that modern shooters are all 6 hour campaigns (if they even have one) and then DOOM comes along with a 12 hour campaign and Bebpo complains.

I agree, the game balance loop is pretty much perfect. The way ammo and killing gets you health and armor, chainsaw gets you ammo, BFG helps in spots, powerups, etc... if you know what's in the room you can strategize. Like even though I'm not too good at FPS since I rarely play them, by saving the powerups/chainsaw/bfgs for the last waves of the toughest enemies (like multiple hell knights) I could just cheeze through them with 1 hit kills when they show up. Made it very doable. The bosses were actually the toughest because they had a different flow.
However, I'll still defend that the game is a bit too long. Ideal game design is that each level adds something new, whether it's a new enemy type, a new weapon type, a new platforming type, etc... to keep things fresh and then the last area or two is just an amalgamation of everything together in a final set of challenge rooms. The thing is, with DOOM, the last enemy types were introduced I think at the very beginning of the crucible (I think the green slime guys or shadow pinkys were the last introduced), and the last half of the crucible was literally nothing but YOU ARE IN HELL, FINAL CHALLENGE ROOMS which combined every enemy in tough intense setups. Then you have the Hellguards. After that the remaining 90 mins or so of the last 3 areas add nothing new and are just the final challenge room type setups that you already spent 60 mins running through in the crucible with nothing new before the Spidermastermind boss. Again, those final 90 mins are still fun because hey, DOOM is sure fun to play, but as I said it felt kinda going through the motions because there was nothing new those last 90 minutes were bringing to the table and the final challenge room before the end boss imo was way easier than some of the earlier challenge rooms. I think if they'd put the Spidermastermind boss at the end of the crucible, the game would have been absolutely perfect from start to finish. As is, it's still great, just a little long. I definitely agree it's the best single player FPS campaign in many, many years.
And there is no ideal length for an FPS campaign. You can have an amazing 4 hour one, 6 hour one, 15 hour one. The campaign should just last however long your content does.
Also, for that bit about the customization, after I beat spider boss I read up on strategies that you were supposed to use on him and it turned out for customization I picked the worst/wrong customization of the two to focus on & upgrade for like every weapon, so I didn't have any of that stuff like remote det rockets or seige mode or mini-turret :'(