That's how I found this.

Back in 2008-10 I started using 10-speed SLX and that was near flawless. Sometime around 2017 I switched to an 11-speed XT (M8000) that I found at a discount. That thing never misses a shift and in addition it can shift under load in either direction without mishifts. Never requires adjustment after a new cable settles. It's incredible. I've gotten really used to this experience.

Recently I got a foldable with M3100. I was pretty excited because the M3100 now has the shadow cage design and it looks more or less like a heavier, cheaper version of what I'm used to. I starter riding it and I got a few misshifts, adjusted the barrel, got a bit better, adjusted the deraileur hanger, got a bit better, B-screw, got better. I even got an old 9-speed XT shifter for it because the derailleur movement is primarily controlled by the shifter. Now it's closer to the consistency I'm used to but it still does some slow shifts / higher-lower shifts every few cassette sweeps. That got me thinking about what could possibly cause this and I started inspecting it physically. That's when I discovered how much play there is in the M3100. I could get the guide pulley to move half a gear from the indexed one by mildly pressing on it. I did the same test on my other bike. It barely moved even when pressing really hard. That's when it hit me that there's no way to get perfectly consistent shifting with this amount of play. It's still perfectly functional, it's not dangerous or anything, but it physically can't do it. I decided to bite the bullet and get some old M8000 gear. Once I removed the chain from the M3100 it was so much more obvious how much play there's in it compared to the M8000.

With all that said, for the price, an M3100 with a cheap trigger shifter is still very good. It just won't ever do perfect shifts and there isn't much point in changing cassettes, cables or shifters in the hope it'll improve significantly.