Painkiller is way too simple and repetitive to be considered that. There is no clever use of space to make different types of battles, the last battle feels and plays exactly the same as the first one. You also don't do anything in those levels except kill stuff until the next area opens. There are a ton of different enemies, but they are basically all the same two types with different skins; one level you fight a knight with a sword, next level it is a WW1 soldier with a bayonet, next it's a motorcyclist with a hammer, it's all the same shit over and over again. Also the game lacks good airborne enemies and bigger enemies in normal battles.
If we are going to talk about retro 2000s shooters, then Serious Sam The Second Encounter wipes the floor with Painkiller. It has larger weapon variety, enemies come in all shapes and sizes with much more varied behavior, it has more complex and better level design with some areas even using physics and gravity in interesting ways. Large scale battles are much more varied with enemies attacking on foot and in air and on several fronts at once. Painkiller doesn't come close.
I don't think comments are accurate at all.
No flying enemies? Then what are these: https://youtu.be/OzMAakLspLU?t=547
There's tons of creative and highly varied different uses of space, i.e this level where you have to carefully tread through a haunted asylum with skulking enemies who you can barely see, and who are unlike the enemies in other levels: https://youtu.be/uXDOwdOYsHk?t=239
By contrast, the first part of this level requires you to use long-range firefight tactics in order be successful: https://youtu.be/KtW9UZS9WLA?t=59
Playing this level with only the stake gun, as is necessary to get one of the Black Tarot cards, is a completely different gameplay experience than anything else in the game: https://youtu.be/_Jfy1A-NJ2E?t=14
All in all, the enemies in Painkiller, have a huge variety, in both appearance and the ways in which they attack you; way better variety than in most FPS games. They are not "reskins" of each other.
Further to address the idea that all the fights are the same: playing on the harder difficulties and going for the goals is quite difficult, and it requires a smart use of tactics, a smart a choice of which Black Tarot cards to equip (and earning each of them in the first place is also one of the goals), a smart choice of using the right ammo vs. the right enemies, a smart choice of when to conserve ammo, a smart choice of when to go for souls or when to let them disappear because it's too risky to go for them, a smart choice of when to or when not to activate Demon Form, a smart choice of when to activate your very limited number of Black Tarot card usages, etc.
In other words, Painkiller is not nearly so simple & dumb as many people often misconstrue it to be.
As for Serious Sam, I played one of them before, and it didn't hold a candle to Painkiller. In Serious Sam, I spent a lot of very boring times wandering around through levels that I'd already cleared out, trying pixel-hunt for the way forward. It was more like doing a chore than playing a game. Plus the monsters & environments & weapons & levels in Serious Sam were way less interesting than those in Painkiller.