

variety of enemies that require different weapons
I had a totally different experience. In my first playthrough I just switched weapons, when I felt like it (and for level challenges), because there’s just no reason otherwise. And the second playthrough was with only the SSG in the first half and Impaler in the second, and they just shredded everything.
I played on default Nightmare and then some custom turbo mode (150% game speed with lower damage taken and dealt, and a bit more).
As the OP said, after a few levels you can get upgrades that destroys shielded enemies quickly and for some reason the game starts spawning mostly superheated ones anyway, so you can just instantly blow those groups up.