So no, ammo is available quite often. But the means to acquire it, and the interruption it causes is annoying, frustrating, stupid, and makes me run around the battle like a headless chicken with a chainsaw, to use the one charge I always have on a zombie. What a heap of trash. Why does this shit have 89% score on Steam?
If you didn't have to rely on the Chainsaw on ammo, you'd have to rely on picking up ammo items in the arena. Which, much like the chainsaw, would force you to disengage from combat and ignore all enemies to find that ammo you really need. The problem is that the circularity of the arenas do not make it immediately obvious where all the ammo is, meaning it would add a layer of trial and error memorization where you die a few times so you can learn where all the ammo is and plan your route around that (and getting rid of arenas as a whole is not an option here). So instead of forcing you to use the Chainsaw, it would force you to hang around these specific parts of the arena where all the ammo is. Meanwhile the benefit of giving your ammo two legs and having it walk towards you is that you do not have to spend as much time trying to find it, as the game routinely spawns fodder enemies as long as heavy demons are still alive. Paradoxically, on average you'd spend less time looking for zombies to chainsaw than looking for ammo items.
And consider the low ammo cap; if the Chainsaw didn't exist, you'd instead be complaining how you have to constantly run around the arena looking for item pick-ups because of how frequently you're out of bullets. Of course you could increase the ammo cap, but that would make it suffer the same problem as nuDoom did where you could just use two weapons for the entire game since you had ammo for days, as you turn your brain off and then call the game boring because it doesn't make you use your brain.
Dev's intention: make player utilize every tool of disposal available, nothing goes to waste.
Result: a combat loop that's trite and predictable, you feel gimped more than powerful enemies pounding your ass.
There's something inherently wrong about a shooter, a genre that often let you express yourself through gameplay, where you have to force players to do this and do that. If there's an underpowered gun or underused tool, the worst balance method you can do is nerfing every other aspect so player is forced to use them. I completed a few levels of Dusk with just the crossbow and I did run out of ammo often, but the game allowed me to do that and I never felt punished for using just it.
What the hell is wrong with just using your favorite guns? In the case of Dusk, I happen to like everything the game threw at me, even the dual pistols have their place, even the sickle is useful for its deflection (thanks Mandalore). That game has no filler, no gimmick aside from the cigarette, every gun is useful while not being tied to this and to that, and it's completely fine. I fucking despise limitations in fast paced FPS, Wolfenstein Youngblood did this shit where these guns are only effective towards these enemies due to their arbitrary armor type, and it was hell to play because that kind of design is tiresome and lazy as fuck.
The idea of "freedom of player expression" more often than not translates to "please let me stick to this one playstyle for the whole game that completely breaks the game but nonetheless lets me live out my power fantasy". And even when you do offer tons of player expression, the average player will more often than not default to the path of least resistance because it works and lets them win the game, no matter how boring said playstyle is. Case in point: Vanquish would let you do a tons of cool shit and is likely the TPS with the single greatest amount of player expression out there, but 70% of players nonetheless played it like a generic cover shooter because a) they had no incentive to do any of the cool shit, b) trying to do cool shit got them killed too often as doing cool shit requires that you're actually good at the game, and c) playing the game like a boring cover shooter lets them progress through the game just fine. nuDoom suffered from the same shit where it didn't really force your weapon usage in a particular way, which then ended up making the game completely repetitive as you had no reason to change your playstyle at any point.
What Eternal managed to do is make using all of your tools essential for survival, instead of games like Devil May Cry where despite all your given tools and the creative focus of the game there's always some kind of exploit that lets you kill enemies more efficiently than others. If you don't learn to use all your tools and stick to your favourite gun like a stubborn mule, you will simply die. The underlying idea here is that not being able to use your favourite weapon for the whole game makes you
think about what to do next, which in turn drives player engagement and gives you more shit to get better at. It's not like the game is a puzzle now where you have to move and kill exactly like this if you want to survive either. The skill floor has now been raised so you're expected to be more competent at a baseline level and utilize all your given tools, but once you do there's plenty of player expression to be had. Just look at the shit the grappling hook lets you do:
The only difference is that you now have to actually work to be able to express yourself, whereas if you want to stick to your favorite weapons for the whole game you either have to swallow your pride and turn down the difficulty, or go play another game.
Weapon-specific weaknesses/resistances are one way of getting the players to play in a more interesting way. For example, in Quake the Shambler has an arbitrary resistance against explosives, because without it you'd just play popamole with the rocket/grenade launcher, so instead you're forced to use either the super shotty or super nailgun to kill it while staying in the open or doing the Shambler Dance, which is a lot more interesting than playing rocket popamole. If the rocket launcher (in any game really) didn't do any arbitrary self-damage, everyone would only ever stick to the rocket launcher instead of using any other weapon because the rocket launcher usually has the best DPS. The Cyber-Mancubus in Eternal has an arbitrary weakness where you can punch its armor off with a Blood Punch and bypass a huge part of its health pool, which encourages you to risk getting very close to an enemy which is specifically designed around not letting you get close. Arbitrary and forced as these mechanics may be, ultimately they're designed around forcing you to think about what you're doing and to make more interesting decisions, instead of unbalanced messes like Dishonored where you basically have to invent your own set of rules and limitations to keep the game interesting. It's refreshing compared to all the recent unfocused sandbox tripe where they want to appeal to as many players as possible, and so add in half-baked stealth mechanics in a shooter or half-baked vehicle sections (or the platforming segments in Eternal, I'm not too happy about those either). To appeal to those who want freedom of expression in their games, you know.
That said, elemental weaknesses type stuff only works if each different element actually functions differently instead of being the same shit but in a different color. It's also a problem if the weaknesses present too perfect of a solution, case in point the Cacodemon grenade swallow for a free Glory Kill which is too easy and too powerful to execute and therefore you should always do it if you have the chance, or the Maurader where you're essentially reduced to one or two options to be able to beat him. OTOH, taking out weakspots on the Mancubi/Arachnotron/Revenant works because sniping their weakpoints is significantly trickier to execute (especially considering it's not like you're always in a situation where you can safely focus completely on just sniping the weakpoint without getting hit in the back) and doesn't completely turn those enemies into a joke if you do.
