Just self installed one on my Canyon, only had a smart trainer to compare it against but the average figures for a 30 min ride with ERG off were within a few watts.
The numbers are baffling, A crank source versus wheel on trainer source complicate it for a start. Everyone has their variations, who’s is the benchmark
Figured £225 and install it myself was a good call for me as a leisure cyclist who certainly can’t justify paying 2 or 3 times that for the big names, who may or may not be any more accurate or last any longer.
Online chat was quick to respond, and they were available on the phone too.
No more disqualifications on zwiftpower for me for using ZP and can use it in the real world too - unlike a smart trainer (although a smart trainer with resistance control is also ideal for zwift).
Give them a look.