Luck stat

[MasterworkStone]

It affects everything from crits to the quality of materials, except hit chance, hit chance is unaffected for balance reasons.

6-18 luck improves everything luck based with diminishing returns, examples: 5 luck - quality 50, 6 luck - quality 90, 7 luck - quality 115, 8 luck - quality 125, ect
5 luck gets you what we have now
4 luck gets you a 50% increase in bad stuff happening to you, bad materials, less crits, more enemy crits
3 luck means nothing good will ever happen, every enemy attack is a critical, every material is it's lowest quality, you will always get your lowest damage possible, flashbangs never incapacitate.

[cypherusuh]

Sounds like a bad stats. Also with current stats, it's already tight enough for some build, it honestly don't need additional stuff.
For quality mercantile technically increased your chance to get good quality. The secret shop adds more stock with better quality. You could reliably found Q140+ on Tier 2 Oligarch shops
For crit chance, it should be handled by Perception. It's logical that you could accurately shoot on fatal point if you're very perceptive, same logic with Dex' melee crit, you could handle melee weapon more accurately and hit fatal point more reliably

[MasterworkStone]

Sounds like a bad stats. Also with current stats, it's already tight enough for some build, it honestly don't need additional stuff.
For quality mercantile technically increased your chance to get good quality. The secret shop adds more stock with better quality. You could reliably found Q140+ on Tier 2 Oligarch shops
For crit chance, it should be handled by Perception. It's logical that you could accurately shoot on fatal point if you're very perceptive, same logic with Dex' melee crit, you could handle melee weapon more accurately and hit fatal point more reliably

Really the idea behind it is for challenge runs, generally people are going to keep it at 5 on regular builds, but for some extra pain they can lower their luck.
16 luck could have a 100% hit chance feat.

[TheAverageGortsby]

Really the idea behind it is for challenge runs

Sounds like a lot of work for something you as the player can easily customize.  Want more challenge? Play through without ever using energy shields.  That one tiny change will profoundly impact the game.  Not enough? Play with no healing item use - you need healed, go to the doctor.  That'll make a huge difference, too.  Still not enough?  Play with slightly-reasonable item stack limits, like one or maybe two grenades per belt slot.  Play an Ironman-style character and reroll when you die.  Play without stealth.  Play with minimal violence.  Play without armor, just run the entire game in your starting clothes.

All of those things require zero dev time, zero mechanics modification, zero changes to the game at all.  And they're already built into the game, ready for you as the player to choose to stick with.  Adding another core stat and figuring out its interactions with the stat balance, skill pools, and game effects while still giving it a useful role to play (because, design 101: If It's In Your Game, It Should Be Meaningful) is an awful lot of work for what amounts to a third or even fourth play-through option.  What do you figure the rate is for that? One player in a hundred?  Personally I doubt its even that high.

Besides, a Luck stat already exists in UnderRail. It's called the RNG 

[Fenix]

In the beginning of development there were more stats - liek 3 more or something according to screenshots.
They were removed to make system more harmonic and flexible I belive.
There is no way back.

[cypherusuh]

Tbh even adding 1 more skill category (TM) kinda breaks the game balance, since now, you could just dump Agi, take Psi and all of its glory and enjoy free, much better sprint, for 55 TM points