To me its not that exhausting to play, unless you play it on higher than Normal. Eternal, especially in the beginning, is incredibly frustrating on Hard with pad controls. To be frank I found 2016 to be more exhausting. Because 2016 almost solely centers around arena battles, with enemy tiers constantly improving the more you slaughter. It gets a bit tedious past the halfway point. Eternal adds more puzzles, platforming and downtime inbetween and I felt the skirmishes were shorter in general. The game, on Hurt me Plenty at least, never really pits 2 barons at you or anything. The weak fodder enemies make the fights look bigger than they actually might be.
What brings Eternal down for me is the excessive amount of platforming and the gimmick battles like the Totems and goo surface. But I'm pretty sure the showdowns are shorter. The Marauder also sucks. I know how to beat him and I can do it in a master level with minions in the mix, but he feels out of place.
About the ammo, I still don't see how this game benefits from it. With more ammo I would also switch up since its more effective to do so. Shields you want to blast with Plasma for free AoE damage, caco's can be quickly glory killed by lobbing an explosive, Spiders are less of a threat if you quickscope their machinegun off, same goes for those fatties. The difference is that I constantly felt forced to retreat to some fodder just to get a few rounds of ammo. The worst offender is the final boss, most I did was running around slicing enemies, shoot about 10 times, repeat.