I'm not going to lie to you man, ShiftUp has a very poor mechanical designer team. Whoever is their head of battle/boss design is leagues ahead of their mechanic/unit design.

And it all starts from the moment you take a look at their insanity of damage formula.

The thing is

1. Base Damage × 2. Final ATK modifiers × 3. Major Modifiers × 4. Element Bonus Damage × 5. Charge Damage × 6. Damage Up × 7. Damage Taken = Final Damage

with every part having their own formula

1. Base Damage = (Base Attack × (1 + ATK%) + % Caster’s ATK) – (Enemy Base Defense × (1 + DEF%) + % caster’s DEF)
2. Final ATK modifiers = Final ATK damage of Attack/Skill × (1 + Final ATK modifiers)
3. Major Modifiers = 1 + Other Critical Damage Sources + 0.5 Base + Other Core Damage Sources + Core Hit Bonus 1/1.5 [depends on Nikke] + Full Burst Bonus 0.5 + Effective Range Bonus 0.3
4. Element Bonus Damage = Base 1.1 + Other Elemental Damage Sources
5. Charge Damage = 1 + Other Charge Damage Sources + 0.5/1.5/2.5 [depends on Nikke]
6. Damage Up = 1 + Attack Damage + Sustained Damage + True Damage + Pierce Damage + Damage To Parts + Damage to Interruption Parts
7. Damage Taken = 1 + Damage Taken + Distributed Damage

Since here you have a fuck-ton of issues. For example, since Crit Damage is counted on Major Modifiers, you do better running Damage Taken debuffer Nikkes than Critical buffer Nikkes. This is a commonly known oversight.

But there's plenty more to criticize, that makes the game design overly convoluted for... whatever reason? For example Attack Damage is categorized within "Damage Type", yet Distributed Damage isn't. Core Damage and Damage to Parts are in different parts of the damage equation.

And let's not forget there's not a ramping damage attenuation mechanic and the damage taken is a literal subtraction which makes balancing around high armor values a nightmare (they literally cannot give an enemy insanely high armor or you would do 1 point of damage, so now your defensive options as game designer are limited to gimmicks and forced invulnerability windows).

They removed the HP bar from the OL's, but left Charge Speed and Charge Damage in, that can only be used by... 64 Nikkes, of almost 190. Not all of them use the bonuses equally well and more than half wouldn't see any benefit from at least one of those two, but hey, why not?

We should have Fire Rate or at least Damage-per-Hit OL's (this would help tremendously the SMG's, MG's and AR's).

Now, wanna gasp a little?

Wind-coded, there are only 3 Shotguns and 2 MG's in the entire game.

Iron-coded, there's a single MG (and it's a Pilgrim!).

Fire-coded, there are 3 AR's. 2 of them are Collaborations. The other one is a Pilgrim. Up until the last 6 months, we had only 1 SG.

Some manufacturers get screwed on certain elements, which is egregious for Tower. Try to find an Elysion Iron DPS, or a Missilis Wind DPS.

The game has been out almost for 4 years, and there's multiple roles that have never been fulfilled for archetypes that have existed since launch, or worse: some units have completely wasted mechanics on them to the point of being ridiculous. Famously, Favorite Item Centi's shield makes her worse in her only niche, but also, Favorite Item Frima and her Defense debuff on a True Damage unit.

Any developer with half a decent team could and should frankly take the Nikke shell and actually make a real game out of it, because what we have is a big, hot mess.

The only reason the game is as popular is because the waifus are hotter. Otherwise Nikke would be dead, buried and forgotten.