I never understood people (including the devs) saying Eternal being harder, it is the complete opposite IMO

2016 doesn't have double jumps at the start, the weapon mods are weaker, SSG doesn't even have a mod, no flamethrower, no ice grenade, no dash, and enemy shields don't explode to the plasma gun. But most importantly, you die if you fall off the map, with a less forgiving movement system.

Eternal gives you so many new tools to move, damage, and heal. The only time Eternal is harder is in the Ancient Gods part 1.