I've got 2 new fat Bikes (Framed Minnesota) with Shimano RD-M6000 rear derailleurs on them. (And another with an RD-M4120.) They're all using Shimano Deore triggers from the same line.
I am having a lot of trouble getting reliable (let also crisp) shifting on the M6000s across the entire 11-42, 10-speed cassette.
Specifically, I'm having trouble on both of them getting the chain to drop reliably from the 18 tooth cog (7th gear) to the 15 tooth (8th gear) (4th cog to 3rd cog, counting from the smallest). This is the main trouble spot when I get the cable tension adjusted for the crispest shifting up the cassette to the bigger cogs. (By crisp, I mean the shift engages quickly right after the shifter clicks, and an immediate release of that paddle keeps the gear.)
With this level of tension adjustment, Shifting up and down works fine on every other move, but it always hangs on that 18-15. When I use the shifter tension adjuster (which only has fairly large detents, like 1/4 of a turn maybe?) to reduce tension to the point where that 18-15 step drops reliably, it means shifting back up the entire cassette gets more difficult.
Specifically, at that looser tension, in order to get an actual shift (and not just a noisy shove against the next larger cog) I must push the lower paddle a tiny bit past the click and hold a bit, waiting for the chain to move up, before I can then relax, and have it hold the cog.
This is not the end of the world: It's not difficult to get the hang of this shift, and it only takes a split second, but it's not really what I expect from a mid-level Deore drive train.
Here's a video I shot showing the performance when it was (mostly) behaving.
(I get better performance with a lot less messing around with the above-mentioned 4120, and also with lower-end Microshift, Deore-compatibles on other bikes.)
I wondered if I were missing something obvious, so I re-installed the mech using the Shimano shop guide, and following that procedure was an unmitigated disaster. It has you adjust SIS tension by shifting into the 2nd cog, then increasing tension until the chain just begins to run against the 3rd cog and make noise. When I do that, the tension is so tight that it won't move down the cassette cogs reliably anywhere. I went back to the method I usually use, which is to mount, check stroke settings inside and out, turn the tension adjuster so I have about 10 clicks to play with, then attach the cable and try shifting from small to big, noting where shifts hesitate, and increasing tension only a click at a time until those shifts work, then testing the other direction -- which is usually just fine at that point.
I installed and adjusted many rear derailleurs that way, and these are the first that I haven't been able to tame.
The performance seems so randomly odd (usually, IME, if tension is too much, the derailleur will hesitate on several cog-drops, not just one.) that I pulled the cable on one bike. One of the pieces of housing was really frictiony, so I replaced it with a smooth piece of brand new PFTE-lined housing, and thought that would be the end of it, but it still wouldn't tune to my expectations.
I've also checked the derailleur hangers to make sure they're straight enough (check) and the small-cog limit screws seem set so that the cage is aligned well. Finally, the cage seems straight enough as checked against a straight edge.
EDIT, video of second (identical) bike exhibiting similar problems even after hanger was measured and adjusted and slicker cable housing was installed.