The mod itself is an odd interpretation. It features some unconventional modifications to the core game's systems.
Here are some of the most prominent changes:
The combination health+stamina system (dubbed Willpower). Your current willpower drops for each action and running out stops you from attacking at all until you can regain some willpower (This will be dubbed "Stamina" from here on out) and if your hit zero stamina it takes longer before the it starts regenerating. Each hit takes away from max willpower temporarily in addition to your current stamina pool, it can only be recovered via using an item or climaxing.
The stamina system also is used in the new struggle system (which needs a tutorial as it's not obvious what to do) so each button press takes away from current stamina and missing a button press (when the bar is pink) will start taking away max stamina and if you let the stages advance too far you'll continuously take max stamina loss. Climaxing applies status effects (as normal in the core game) and resets your maximum willpower (What? Why?) and raises corruption (permanent stamina loss, minus a game reset via new game+). The struggle system easily slows down the game by several magnitudes since you have to build momentum and spend stamina while doing it and it can take a long while before you can resolve the struggle only to get grabbed by the next enemy in line. Many a times I just reload as soon as the struggle mini-game appears or just eat the cost because it's that un-fun. Some bosses like the Doomsday Device or the magnetic boss are just pure cancer to fight due to the chain grabs. Also note that when you escape from a failed struggle you start at zero stamina after recovering unless negated with equipment.
Attacks take differing amount of stamina. A ground attack is 1.2 bars of stamina. A jump attack is 2.5 bars of stamina. Ground pounds are 4 bars of stamina. Keep in mind you only have 14 bars of stamina at max (zero damage taken).
Base stamina regeneration is also extremely slow. For you Dark Souls players in the back there the regeneration rate is roughly the same as someone constantly blocking. For the Monster Hunter crowd it's being affected by Water Blight. I have spent more time sitting around doing nothing just to have enough resources for the next encounter. It takes almost 15 seconds to recover stamina from zero. It takes 3 seconds to start recovering from zero stamina. This makes the janky bullet deflection mechanic not worth attempting since you have a high chance of wasting stamina if the deflected bullet flies into the cosmos instead of back at the enemy. It also makes fighting any boss a royal pain in the ass. Mari is probably the worst offender since you have to break her shield. I have had to sit there with a thumb up my own ass waiting for stamina to regenerate, wasting perfectly good openings to deal damage after the shield drops because the mod demands that you play perfectly.
The stamina system makes an already buggy game even buggier. If you get caught on enemies trying to jump and have zero stamina you are effectively stuck until you gain enough to attack again. In the final stages of the game where you are hounded by many of the slave soldiers who have bullshit insta-blocking abilities and can only be damaged when doing any other action, you'll get body-blocked on the regular. I've been pushed out of the terrain a few times due to that.
You can no longer combo attack normally. You need a piece of equipment to re-enable combos again. A full combo takes 2 bars of stamina.
Speaking of equipment, equipment stats are rolled. You should aim to be rolling with max fragments as often as possible to minimize the detriments and to maximize benefits. Some of these are game-changing (recover max stamina on kill) and are reliant on RNG to an extent. There is a maximum number of equipment slots, attempting to roll beyond this limit bugs it out and is essentially wasted. You need to pray for some decent RNG for your first set.
The transformation grants projectile immunity and nothing else unless you have the appropriate gear. It's an instant escape but doesn't do anything otherwise.
Some status effects have additional effects. The Hack Visor in particular has an interesting mechanic associated with it if not removed. If a status goes red, hitting 100% will result in a game over. It's also extremely buggy.
It costs 1k to return to base after beating 1-2's boss.
The normal mook gunners now have a separate shotgun blast that can fire off at the same time as their normal blast and (I swear) can fire 180 degrees in the wrong direction, including straight up. They immediately follow this up with the capture grenade. It's enough time to wombo combo you into an inescapable struggle game (remember that damage also decreases your stamina in addition to max stamina). The shotgun blast is a 100% knockdown except during a ground pound which has super armor. The grenade is also seeking so they can yeet the grenade across the map if you moved far enough. The positioning and density of these mobs tells me that it was a mechanic implemented to stop people from just rushing from point A to B. Strangely the Slave Visor enemies encountered in Chapter 2 don't have this shotgun blast and are much easier in comparison.
This is Dark Souls Scholars of the First Sin Master Quest: I Wanna Be The Guy Edition. Ironically the final boss is really easy compared to the gauntlet of bullshit along the way. This is like facing Smelter Demon after going through the hell that is Iron Keep.
Edit 1: Added in stamina recovery information
Edit 2: A note that this is probably the most stressful tutorial mission I have undertaken. It's almost Shrine of Amana levels of cancer.
Edit 3: What isn't cool either is having no invuln time between getting grabbed. It becomes a cascading loss if you are surrounded by enemies or if enemies decide to chain bullshit moves back to back. You're better off just restarting the level or in the case of a lot of people, watching the opening cutscene again.
Edit 4: What's going to be a huge issue are the enemies hiding in the foreground getting cheapshots at you. You're already strapped for resources and that just piles on the stress. I can only imagine what hell the Abyssal difficulty will be.
Edit 5: As I go through 1-1 for the 20th time, the enemy positioning is particularly devious since you can no longer slap the jelly fish out of the air I've had a situation where I was bounced back and forth 5 times getting crippled in less than 5 seconds.
Edit 6: Added in additional mechanics and finds.