Doing Frozen Wilds early gives a huge boost. You get real upgrades (stats, not slots) to 3 key weapons and unique triple stat mods to support them. The rest of the power comes from random drop purple mods, which can roll 3 stats as well; usually the third stat for weapons is a throwaway (some low %elemental damage) but the 3 stat mods have higher base rolls, so you can get things like 67% handling + 25% damage, 50% damage + 25% tear, etc. Triple stat armor mods are arguably even better; something like 20% resist melee, 16% resist fire, 7% resist ranged. You can put 3 or 4 (new game+) of those in an armor.
The problem is these mods are very rare and you will only gather a handful in a 100% playthrough unless you go out of your way to kill big machines frequently.
Then the other half of the equation is what wormie and Vorph said. Eventually each machine becomes a minimum resource formula + proper initiation, and the variation comes when you miss or have to fight a mix. In Ultra Hard I've stopped using tearblast arrows altogether, and each kill is now 1-2x fire/shock or 3-6x frost, and a certain number of hardpoint or precision arrows.
Overall I find it much more satisfying than the typical RPG power creep. You never become tanky enough to where taking a hit is a viable alternative to dodging, but even so the interruption of a charged bow shot is more devestating than the damage itself.
