Willwrote a review of Pioneer Shimano Ultegra R8000 Single Leg Power Meter Crank Arm on December 4, 2018
Familiarity: I've used it once or twice and have initial impressions
Used this power meter a couple of times now. Seems to be working properly and is accurate. A few notes that may help others: on the install instructions included with the unit, there was a section for installation without the magnet. This is what I attempted as I use a duotrap for speed and cadence. The unit will not work without the magnet. I read a few places that with a firmware update it will work, but I use a Garmin Edge, so I cannot verify.
Another note: I installed on a Trek Emonda SL6, and after the first couple of rides I could see, upon very close inspection, a small rub mark on the chainstay. I don't think it's anything to worry about. most people probably wouldn't even notice it, but there are small 0.5 and 1 mm shims available for the crank arm (they should really include a couple of these for the price of power meters, but whatever). I'll probably end up putting one on at some point, although any rubbing that happened has probably worn off enough material to not rub anymore... I may just leave it. Power numbers look good and seem to be accurate. it's cold out now, so I'll keep track of it as temps get warmer in the coming months.
For some reason, this shipped to me USPS Priority and it took almost a calendar week to get it. Ordered on Sunday, received the next Saturday.. which in this day and age of 2 day and overnight everything is disappointing. But Backcountry/competitive are good people and have always done me right, so still highly recommended.