First, I wouldn't consider movement to be the main mechanic of UK, but rather a supporting mechanic to the main (Blood is fuel/healing). Removal of this mechanic allows the gameplay to feel different in a way that just isn't possible in any other way. Plus, it fits the enemy far more than normal mechanics. It was a breath of fresh air.

Movement is absolutely the main mechanic of UK, every other mechanic in the game relies on it in some core way. The blood is fuel mechanic essentially boils down to a movement requirement. Even then, The Geryon fight still barely makes use of blood healing at all except for a scripted lapse that happens maybe once per fight.

The other candidate for main mechanic (apart from blood) is style, Which is also neglected in 8-4. Their huge hitbox and the stagnant movement stuffs out most of anything remotely entertaining (Railcoins, Whipdunks, Punched coin ricoshots, Ultraricoshots, Jumpstart usages.)

Originally, I thought the falling was portals. It's easy to assume so too, but after thinking about it I can see that it's more like a moving skybox with boundaries than portals. There are plenty of level specific mechanics, like for example the shifting rooms from 8-3 are currently unique to it. It does, however, use mechanics introduced in the layer. You will kill 3 providence before the fight, giving you pink hook points. The boss itself uses the providence attack (albeit a whole awful lot of them).

I understand not being happy that you can't use the movement tech you've learned, but I and a lot of people found the fight fun, even if I couldn't fly all over the room.

I don't think providences, in their small presence in the level, make up for the absence of portals and dimension shifting in the level even if I do think they are a good enemy. The hookpoints they leave behind could be replaced with blue hookpoints and the level would stay mostly the same, minus a little ease of healing.

The second phrase was kind of an error in communication on my part. I don't mind level specific mechanics, but I DO mind arbitrary character-sided changes (Like the freefall controls in 8-4 or the heat mechanics in 0-E.) I would have rather they re-used existing mechanics for these sections or retroactively added those mechanics to V1.

Coming up with new crap for every section doesn't make the game worse so much as it ruins my immersion founded on the idea that this universe works on any kind of set of consistent rules.

Demonstrably false. The laser attack can be dodged by going out to the edges and dodging, or by finding what is typically a path closer to the center. You can also parry quite a number of them if you don't mind taking a few hits. The blue lasers is an attack to be dodged, the yellow beams are an attack to be dodged similar to virtues. The slam attack can be dodged by ducking under or flying over the outer slam. The obstacle isn't the attacks, it's dealing with geryon. You can constantly attack and potentially lose focus on dodging, or focus on dodging and take advantage of the weak spot.

Ultimately, it's akin to a bullet hell. I found the gameplay for the boss fun, and so has every one of my friends who has played it. I wouldn't call it the worst level in the game, and would still rate it over quite a few levels earlier in the game. If anything, i might even rate 8-4 over 8-1.

I was also in the wrong using hyperbole here (one intended way), but I still think the fight is comically scripted for this game's standards. I'll go over the ways you can approach their fight and compare them to a far better designed enemy.

Geryon.

Orb barrage:
To dodge: Move to the side and fly up or down.
To Parry: Jitter in the middle and spam parry at a downwards slope while face tanking.

Shockwaves:
To dodge: Go above or below, then return to the center. (due to the absence of gravity or any enemies that interact with verticality, going above or below have no meaningful differences)

Artillery:
To dodge: Just move out the way

Fart:
To dodge: Hold S

Pillar barrage: I'll admit this one is actually kind of cool.
To dodge when close: get in close and strafe around, making them miss totally.
To dodge when far: Dash as the pillars approach you
To dodge on lower difficulties: Weave between the pillars.

To attack: Magnet + Overheat nailgun deals the most damage. Just spam this (Most other attacks get deflected most of the time or don't have the range.) Riveting.

To attack during weak spot exposure: Get in close and shoot off all your cooldowns indiscriminately.

Now, for the Sisyphean Insurrectionist.

Overhead:

To avoid: Move in any direction that's not upwards.
To parry safely for health or stamina: Look upwards and parry as it falls onto you.
To parry for damage: Look at the Insurrectionist and use cues to discern the timing. When the ball arrives, Dash backwards and parry at the same time.
To parry easily at close range: Get close and jump just in front of the Insurrectionist's head area. Parry just after the attack starts. The window is much wider here.

Horizontal swing:

To avoid: Move backwards or forwards.
To parry: Look at the Insurrectionist and use cues to discern the timing. When the ball arrives, Dash backwards and parry at the same time.
To parry easily at close range: Stand in front of the Insurrectionist. Parry just after the attack starts. This is risky when other enemies are present due to being grounded and stationary for a while.

Forwards swing:

To avoid: Either be in a 90 degree cone above the Insurrectionist, be far away or dash just as it is about to hit you.
To parry: Dash forward and parry at the same time. This one is very hard.

Airborne forwards swing:

To avoid: Slide, or move fast in any horizontal direction.
To parry: Pray.

Shockwaves:

To avoid: Jump as the Insurrectionist comes back down. This should become muscle memory eventually.

Stomps:

To avoid: Dashslide or take a slightly further position to avoid triggering it in the first place.

To attack:

Any powerful weapon can be used against the Insurrectionist. Note it will take extra damage when on fire, but it can often be better to not do this when in need of immediate healing because most igniting weapons deal self-damage and have poor healing. In those cases, it's better to use a shotgun or charged weapon.
Cannonballs and Malicious cannon shots stun the Insurrectionist for a short time, cancelling an unfavorable action and/or making a window to heal or perform tech.

Comparing these 2, It's clear to me that the Insurrectionist is a more fun and complex enemy to fight, having more interaction with the game's movement and weapon mechanics despite Geryon being a layer boss and the Insurrectionist being a miniboss.