Just to be clear, I never, ever said it's like that.
On the contrary, I explicitly praised the level design and the exploration.
And also, even when you fight a big amount of enemies till you've killed them all, as I wrote you are basically never inside a simple square room, but you do have lots of freedom of movement, verticality, hallways, corridors and hideouts.
You always move around, enemies move too, you need to search for powerups, shields, energy and bullets in order to survive, never standing still.
So does it feel at times quite a bit like an arena, considering that enemies keep spawning until you are the sole survivor? Yes, indeed it does, in my experience.
Is the game just a series of rooms full of enemies connected by hallways? No, it's not.
The weapons are heavy, mean and absolutely a blast to use. And it's impossible not to love them in general, considering the damage they cause to those poor infernal creatures.