THIS SUCKS
Rationale:
1) No good way to visually communicate item quality
Whether it's dots or an item glow, the game needs to communicate that a Common item is different from a Legendary item. The current dots are UI leaking into the game world, which is a design hack.
I realize this is with overlay on. With overlay off, a Legendary inserter looks the same, but acts differently.
Cue PVP salt about legendary buggies with legendary bullets beating common spidertrons.
Cue "You should've hovered to see that's a LEGENDARY buggy and you have no chance"
2) Parallel mechanism to tiered machines
Machine efficiency is a two-dimensional matrix - machine tier vs modules. Now WUBE is adding a third axis. What's better, a legendary T2 assembler or Common T3 assembler?
Might be particularly frustrating with inserters.
Current behavior:
T0: Yellow inserter 10 u/m
T1: Blue inserter: 15 u/m
New behavior:
Yellow common inserter 10 u/m
Yellow rare inserter 12 u/m
Yellow epic inserter 14 u/m
Blue common inserter: 15 u/m
Yellow legendary inserter: 16 u/m
Blue rare inserter: 17 u/m
QUALITY does what ITEM TIERS do, but worse.
...
3) Possible optimization impact
Megafactories are possible because a compressed kilometer long belt of circuits is optimized into a single structure. On that compressed belt, the ordering doesn't matter - just item count. The freshly created circuit can magically teleport itself to the end of the belt and everything behaves the same. Not so if there's item quality.
4) Quality doesn't make sense for some items
Some items benefit greatly from quality, some do not. I'm expecting that 10-20% of items players will want in maximum quality, 80% don't care.
But quality applies to everything, it's a generic mechanism. Devs will be stuck trying to figure out what quality does to an item, potentially implementing a LOT of extra stuff.
5) Avoids reciple bloat, but not really
The design goal of item quality was to avoid recipe bloat, and that's checked on the surface level. Underneath though, if you account for quality in efficiency calculators, I expect the number of numerically unique recipes (ie. common plastic + epic red circuit + common wire + legendary wire + epic wire + epic wire) to become astromonical.
Factorio calc devs will have to do some advanced maths I feel, brute-forcing quality calculations may be computationally infeasible.
6) Breaking existing designs
Existing factory designs may rely on fine calibration and clever behavior. A higher tier version of an item might be faster, but that might result in breaking existing designs that are built for specific inserter speed, for example. Designs sensitive to this quality difference may demand specific quality components - which sucks, considering that quality, unlike existing item tiers, cannot be well communicated.
Cue "Finally found out why my uranium setup broke. My inserter was rare quality!"
7) Bethesda problem of Lesser+ Potion of Fire Resistance
Why do I have my inventory full of potions of fire resistance of varying qualities, giving anything from +15 to +60% fire resistance? Does having 4 potions of fire resistance of different qualities really add to the game?
8) Quality tier naming
That is, most obviously a duck.
https://www.simplethread.com/looks-great-lose-the-duck/
Unless WUBE plans to genuinely get a LLM to write a unique short legend for each Legendary Iron Gear Wheel.
RECOMMENDATION
REVERT NOW BEFORE IT'S TOO LATE
I'm fine with quality modules. I'm fine with special versions of items, but they should be unique items, with unique graphics.
4 tiers is too many. 1-2 should be enough, but quality version count ought to be item-specific.
Only select items, for which quality makes sense, should have high quality versions.