MTB Di2? No, I have only had loaners with Di2 on road bikes. (Didn’t like the ergonomics of the levers.)
I currently have M9000/M8000 mechanical (i. e. 11-speed) on my mountain bike (XTR trigger shifter, formerly XTR rear derailleur, now XT). And I have had Shimano XT drivetrains on my mountain bike going back till 2005ish.
FWIW, I have been super happy with mechanical Shimano MTB drivetrains. Just saying that in my experience, I can’t shift under full load without the drivetrain complaining. Sometimes shifts simply won’t be actuated until I let off power, other times, the chain “snaps” to the next gear (but it doesn’t snap, of course). Since I have been trained for decades, I simply let off for a small moment when shifting.
On a mountain bike you often need to spike power to overcome obstacles (e. g. roots and rocks), so power spikes are more common.
I have never needed to do the same on a road bike, so that was never a concern of mine on Shimano (Ultegra/105) or, currently, SRAM Force D1 AXS eTap drivetrains.